U.S. patent application number 16/355921 was filed with the patent office on 2019-09-19 for pressure-sensitive adhesive sheet and magnetic disc device.
The applicant listed for this patent is NITTO DENKO CORPORATION. Invention is credited to Kenji FURUTA, Akira HIRAO, Tatsuya SUZUKI.
Application Number | 20190284445 16/355921 |
Document ID | / |
Family ID | 67905224 |
Filed Date | 2019-09-19 |
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United States Patent
Application |
20190284445 |
Kind Code |
A1 |
FURUTA; Kenji ; et
al. |
September 19, 2019 |
Pressure-Sensitive Adhesive Sheet and Magnetic Disc Device
Abstract
Provided is a PSA sheet that has a high level of moisture
resistance while maintaining good holding power with reduced gas
emission. The PSA sheet provided by this invention comprises a
moisture-impermeable layer and a PSA layer provided to one face of
the moisture-impermeable layer. The PSA layer comprises a polymer A
having a weight average molecular weight of 5.times.10.sup.4 or
higher as a base polymer and a polymer B having a number average
molecular weight of 1000 or higher and a weight average molecular
weight lower than 5.times.10.sup.4.
Inventors: |
FURUTA; Kenji; (Osaka,
JP) ; HIRAO; Akira; (Osaka, JP) ; SUZUKI;
Tatsuya; (Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NITTO DENKO CORPORATION |
Osaka |
|
JP |
|
|
Family ID: |
67905224 |
Appl. No.: |
16/355921 |
Filed: |
March 18, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C09J 7/201 20180101;
C09J 7/381 20180101; C09J 2423/00 20130101; C09J 2421/00 20130101;
G11B 5/012 20130101; C09J 123/20 20130101; C09J 7/385 20180101;
C09J 123/22 20130101; C09J 133/04 20130101; C09J 7/401 20180101;
G11B 21/106 20130101; C09J 2433/00 20130101; C09J 2423/00 20130101;
C09J 2433/00 20130101; C09J 2421/00 20130101; C09J 2423/00
20130101; C09J 123/22 20130101; C08L 23/20 20130101; C09J 123/20
20130101; C08L 23/22 20130101; C09J 133/04 20130101; C08L 23/20
20130101 |
International
Class: |
C09J 7/38 20060101
C09J007/38; C09J 7/20 20060101 C09J007/20; G11B 21/10 20060101
G11B021/10; G11B 5/012 20060101 G11B005/012 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 19, 2018 |
JP |
2018-051399 |
Claims
1. A magnetic disc device comprising at least one data-recording
magnetic disc, a motor that rotates the magnetic disc, a magnetic
head that at least either reads or writes data on the magnetic
disc, an actuator that moves the magnetic head, and a housing that
houses the magnetic disc, the motor, the magnetic head and the
actuator; wherein the housing is provided with a cover seal, the
cover seal is a pressure-sensitive adhesive sheet comprising a
moisture-impermeable layer and a pressure-sensitive adhesive layer
provided to one face of the moisture-impermeable layer, and the
pressure-sensitive adhesive layer comprises a polymer A having a
weight average molecular weight of 5.times.10.sup.4 or higher as a
base polymer and a polymer B having a number average molecular
weight of 1000 or higher and a weight average molecular weight
lower than 5.times.10.sup.4.
2. The magnetic disc device according to claim 1, wherein the
housing comprises a box-shaped housing base member having a top
opening and a cover member to cover the opening.
3. The magnetic disc device according to claim 2, wherein the
housing base member has a recessed portion inner-circumferentially
around the top opening and the cover member has an outer rim placed
on the bottom of the recessed portion.
4. The magnetic disc device according to claim 1, wherein the cover
member has a hole.
5. The magnetic disc device according to claim 1, wherein the
pressure-sensitive adhesive sheet seals the internal space of the
magnetic disc device.
6. The magnetic disc device according to claim 1, wherein the
pressure-sensitive adhesive sheet covers and seals the top face of
the housing base member of the magnetic disc device.
7. The magnetic disc device according to claim 1, capable of
heat-assisted magnetic recording.
8. A pressure-sensitive adhesive sheet comprising a
moisture-impermeable layer and a pressure-sensitive adhesive layer
provided to one face of the moisture-impermeable layer, wherein the
pressure-sensitive adhesive layer comprises a polymer A having a
weight average molecular weight of 5.times.10.sup.4 or higher as a
base polymer and a polymer B having a number average molecular
weight of 1000 or higher and a weight average molecular weight
lower than 5.times.10.sup.4.
9. The pressure-sensitive adhesive sheet according to claim 8,
wherein the number average molecular weight of the polymer B is
2000 or higher.
10. The pressure-sensitive adhesive sheet according to claim 8,
wherein the polymer B is at least one species selected among
olefinic polymers and diene-based polymers.
11. The pressure-sensitive adhesive sheet according to claim 8,
wherein the polymer B is a polybutene.
12. The pressure-sensitive adhesive sheet according to claim 8,
wherein the polymer A is at least one species selected among
rubber-based polymers and acrylic polymers.
13. The pressure-sensitive adhesive sheet according to claim 8,
having a moisture permeability lower than 30 .mu.g/cm.sup.224 h in
the in-plane direction of bonding area of pressure-sensitive
adhesive sheet, determined at a permeation distance of 2.5 mm based
on the MOCON method.
14. The pressure-sensitive adhesive sheet according to claim 8,
having an amount of thermally release gas of 10 .mu.g/cm.sup.2 or
less, determined at 130.degree. C. for 30 minutes by gas
chromatography/mass spectrometry.
15. The pressure-sensitive adhesive sheet according to claim 8,
used for sealing the internal space of a magnetic disc device.
16. A release liner-supported pressure-sensitive adhesive sheet
comprising the pressure-sensitive adhesive sheet according to claim
8 and a release liner protecting the adhesive face of the
pressure-sensitive adhesive sheet wherein the release liner is a
non-silicone-based release liner free of a silicone-based release
agent.
17. A magnetic disc device comprising the pressure-sensitive
adhesive sheet according to claim 8.
18. The magnetic disc device according to claim 17, wherein the
pressure-sensitive adhesive sheet seals the internal space of the
magnetic disc device.
19. The magnetic disc device according to claim 17, wherein the
magnetic disc device has a housing base member and the
pressure-sensitive adhesive sheet is a cover seat that covers and
seals the top face of the housing base member.
20. The magnetic disc device according to claim 17, capable of
heat-assisted magnetic recording.
Description
CROSS-REFERENCE
[0001] The present invention claims priority to Japanese Patent
Application No. 2018-051399 filed on Mar. 19, 2018; and the entire
content thereof is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] The present invention is related to a pressure-sensitive
adhesive sheet and a magnetic disc device comprising the
pressure-sensitive adhesive sheet.
2. Description of the Related Art
[0003] In general, pressure-sensitive adhesive (PSA) exists as a
soft solid (a viscoelastic material) in a room temperature range
and has a property to adhere easily to an adherend with some
pressure applied. For such a property, PSA is widely used in a form
of, for instance, an on-substrate PSA sheet having a PSA layer on a
support substrate, for purposes such as bonding, fastening,
protection and sealing in various applications including electronic
devices. For instance, technical literatures related to PSA sheets
that air-tightly seal internal spaces of magnetic disc devices
include Japanese Patent Application Publication Nos. 2014-162874,
2017-014478 and 2017-160417. In this application, because the
allowable maximum temperature is limited, PSA that does not require
heat for press-bonding is preferably used as the bonding means.
SUMMARY OF THE INVENTION
[0004] For instance, the conventional PSA sheets all comprise
non-breathable substrates and are used in magnetic disc devices
such as hard disc drives (HDD), in embodiments to seal their
internal spaces where magnetic discs (typically HD) are contained.
In particular, a void space that can be present between a cover
member and a housing base member in which the magnetic disc is
placed can be covered and sealed with a PSA sheet so as to obtain
air-tightness for the internal space of the device. Such air-tight
properties may be essential and particularly important in a type of
device whose internal spaces is filled with a low-density gas such
as helium in order to reduce the influence of air flow generated by
the spinning disc. In an embodiment using the PSA sheet, the
sealing structure can be made thinner than in a conventional
magnetic disc device for which air-tightness has been assured with
a gasket; and therefore, this embodiment is advantageous in
increasing the density and capacity of a magnetic disc device. This
embodiment does not require use of a liquid gasket. Thus, it can
mitigate outgassing (gas emission) problems due to gasket.
[0005] Lately, to further increase the capacity, studies are
underway on magnetic disc devices using HAMR (heat-assisted
magnetic recording). In short, HAMR is a technology that uses a
laser beam to increase their surface recording densities. In this
technology, the presence of internal moisture attenuates the laser
beam and badly impacts on the recording life (the number of times
that it can be overwritten). Thus, it is desirable to exclude
moisture from HAMR as much as possible. With regard to this, in
Japanese Patent Application Publication Nos. 2017-014478 and
2017-106417, the cup method is used to evaluate the moisture
permeability of a PSA sheet having an aluminum layer. However, the
moisture permeability test of a PSA sheet by the cup method has
been unsatisfactory to quantify a minute amount of water vapor
transmission which may affect HAMR.
[0006] Accordingly to further enhance the water vapor-blocking
properties, (or moisture resistance) of PSA sheets, the present
inventors have conducted studies including test methods for
moisture permeability to identify moisture permeable channels
through the PSA sheets and established a novel, effective method
for testing moisture permeability (a method for accurately
assessing through-bonding-plane moisture permeability i.e. moisture
permeability in the in-plane direction of bonding area (bonding
interface)). As a result of researching countermeasures for
moisture permeation based on the novel test method, a PSA sheet
having a high level of moisture resistance capable of extending
HAMR life and the like has been successfully developed, whereby the
present invention has been completed. In other words, an objective
of the present invention is to provide a PSA sheet that has a high
level of moisture resistance while maintaining good holding power
with reduced gas emission. Another objective of this invention is
to provide a magnetic disc device using the PSA sheet.
[0007] The present description provides a PSA sheet comprising a
moisture-impermeable layer and a PSA layer provided on one face of
the moisture-impermeable layer. The PSA layer comprises, a polymer
A having a weight average molecular weight (Mw) of 5.times.10.sup.4
or higher as a base polymer and a polymer B having a number average
molecular weight (Mn) of 1000 or higher and a weight average
molecular weight lower than 5.times.10.sup.4.
[0008] The PSA sheet thus constituted can prevent moisture
permeation in the PSA sheet's thickness direction with the
moisture-impermeable layer as well as moisture permeation in
in-plane directions of bonding area (in directions perpendicular to
the thickness direction of the PSA sheet) while maintaining good
holding power. In particular, with the use of polymer A (a
high-molecular-weight polymer with Mw.gtoreq.5.times.10.sup.4) and
polymer B (a low-molecular-weight polymer with
Mw<5.times.10.sup.4) having a Mn of 1000 or higher, while a
practical level of holding power is maintained, moisture permeation
in in-plane directions of bonding area of PSA layer can be greatly
prevented. The use of polymer B with Mn.gtoreq.1000 can limit the
amount of outgassing as well. For instance, when the PSA sheet
disclosed herein is used as a sealing material in a magnetic disc
device, while good sealing properties are maintained based on the
holding power, it is possible to greatly limit changes (typically
increases) in internal humidity that may affect the normal and
highly precise operation of the device. In addition, internal gas
contamination can also be limited to or below a certain level.
[0009] In a preferable embodiment of the PSA sheet disclosed
herein, the polymer B has a number average molecular weight of 2000
or higher. In this embodiment, the decrease in holding power can be
further reduced while the moisture resistance is further enhanced.
The amount of outgassing tends to further decrease as well.
[0010] In a preferable embodiment of the PSA sheet disclosed
herein, the polymer B is at least one species selected among
olefinic polymers and diene-based polymers. With the use of the
olefinic polymer and/or diene-based polymer as the polymer B,
through its hydrophobic and nonpolar character as well as the
effect to prevent moisture permeation based on the molecular
weights in the prescribed ranges, excellent moisture resistance can
be preferably obtained. In particular, the polymer B is preferably
a polybutene.
[0011] In a preferable embodiment of the PSA sheet disclosed
herein, the polymer A is at least one species selected among
rubber-based polymers and acrylic polymers. The use of rubber-based
and/or acrylic PSA layer can preferably combine moisture resistance
and reduction of gas emission.
[0012] In a preferable embodiment of the PSA sheet disclosed
herein, the PSA sheet has a through-bonding-plane moisture
permeability lower than 30 .mu.g/cm.sup.224 h when determined at a
permeation distance of 2.5 mm based on the MOCON method. The PSA
sheet satisfying this feature is highly moisture-resistant; and
therefore, it can be preferably used for a purpose for which the
presence of moisture is undesired.
[0013] In a preferable embodiment, the PSA sheet disclosed herein
has an amount of thermally released gas of 10 .mu.g/cm.sup.2 or
less when determined at 130.degree. C. for 30 minutes by gas
chromatography/mass spectrometry (GC-MS). In the PSA sheet
satisfying this feature, the amount of thermally released gas is
greatly limited; and therefore, internal gas contamination can be
greatly limited.
[0014] The PSA sheet disclosed herein has good holding power as
well as excellent moisture resistance with reduced gas emission.
Thus, it is preferably used for sealing the internal space of a
magnetic disc device where entry of moisture and gas needs to be
limited. The art disclosed herein provides a magnetic disc device
comprising a PSA sheet disclosed herein. The PSA sheet may serve to
seal the internal space of the magnetic disc device. In the
magnetic disc device in such an embodiment, the PSA sheet is
relatively thin, yet provides moisture resistance and airtight
properties; and therefore, as compared to a conventional
gasket-type product, the capacity can be further increased with a
lower amount of released gas. In particular, with the use of the
PSA sheet disclosed herein in a HAMR magnetic disc device, a
magnetic recording device having a higher density can be
obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows a cross-sectional diagram schematically
illustrating an example of constitution of the PSA sheet.
[0016] FIG. 2 shows a schematic diagram illustrating the method for
determining the moisture permeability.
[0017] FIG. 3 shows an enlarged top view of a sample used in
determining the moisture permeability.
[0018] FIG. 4 shows a cross-sectional diagram schematically
illustrating the magnetic disc device according to an
embodiment.
[0019] FIG. 5 shows a cross-sectional diagram schematically
illustrating the magnetic disc device according to another
embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0020] Preferable embodiments of the present invention are
described below. Matters necessary to practice this invention other
than those specifically referred to in this description can be
understood by a person skilled in the art based on the disclosure
about implementing the invention in this description and common
technical knowledge at the time the application was filed. The
present invention can be practiced based on the contents disclosed
in this description and common technical knowledge in the subject
field. In the drawings referenced below, a common reference numeral
may be assigned to members or sites producing the same effects, and
redundant descriptions are sometimes omitted or simplified. The
embodiments described in the drawings are schematized for clear
illustration of the present invention, and do not necessarily
represent the accurate size or reduction scale of an actual product
of the PSA sheet or magnetic disc device of this invention or of
the moisture permeability tester.
[0021] As used herein, the term "PSA" refers to, as described
earlier, a material that exists as a soft solid (a viscoelastic
material) in a room temperature range and has a property to adhere
easily to an adherend with some pressure applied. As defined in
"Adhesion Fundamentals and Practice" by C. A. Dahlquist (McLaren
& Sons (1966), P. 143), in general, PSA referred to herein can
be a material that has a property satisfying complex tensile
modulus E* (1 Hz)<10.sup.7 dyne/cm.sup.2 (typically, a material
that exhibits the described characteristics at 25.degree. C.).
[0022] The concept of PSA sheet herein may encompass so-called PSA
tape, PSA labels, PSA film, etc. The PSA sheet disclosed herein can
be in a roll or in a flat sheet. Alternatively, the PSA sheet may
be processed into various shapes.
<Constitution of PSA Sheet>
[0023] The PSA sheet disclosed herein can be, for instance, an
adhesively single-faced PSA sheet having a cross-sectional
structure as shown in FIG. 1. A PSA sheet 1 comprises a
moisture-impermeable layer 10 and a PSA layer 20 supported on a
first face of moisture-impermeable layer 10. In particular,
moisture-impermeable layer 10 is a layered body (laminate film) in
which a first resin layer 12, an inorganic layer 14 and a second
resin layer 16 are layered in this order. The first resin layer 12
placed on the first face side of inorganic layer 14 forms an outer
surface of PSA sheet 1 while the second resin layer 16 is placed on
the second face side of inorganic layer 14, that is, the PSA layer
20 side. From the standpoint of the moisture resistance, PSA layer
20 is formed continuously over the entire first face of
moisture-impermeable layer 10 at least in the area that bonds to an
adherend. PSA sheet 1 prior to use (before applied to the adherend)
may be protected with a release liner (not shown in the drawing)
having a release face at least on the PSA layer 20 side
surface.
<PSA Layer>
(Polymer A)
[0024] In the art disclosed herein, the type of PSA forming the PSA
layer is not particularly limited. The PSA may comprise, as its
base polymer (polymer A), one, two or more species of various
rubber-like polymers such as rubber-based polymers, acrylic
polymers, polyester-based polymers, urethane-based polymers,
polyether-based polymers, silicone-based polymers, polyamide-based
polymer and fluorine-based polymers that are known in the PSA field
from the standpoint of the moisture resistance and reduction of
outgassing, it is preferable to use a rubber-based PSA comprising a
rubber-based polymer as the polymer A for a PSA comprising an
acrylic polymer as the polymer A. Other examples include a PSA
comprising a rubber-based polymer and an acrylic polymer as the
polymer A. In particular, a highly moisture-resistant rubber-based
PSA layer is more preferable. When the PSA sheet disclosed herein
is used in a magnetic disc device, it is desirable that the PSA is
essentially free of a silicone-based polymer which may form
siloxane gas.
[0025] The PSA sheet having a rubber-based PSA layer and the PSA
sheet having an acrylic PSA layer are primarily discussed below;
however, the PSA layer of the PSA sheet disclosed herein is not
limited to a layer formed of a rubber-based PSA or an acrylic
PSA.
[0026] As used herein, the "base polymer" of a PSA layer refers to
one, two or more species of rubber-like polymer with
Mw.gtoreq.5.times.10.sup.4 forming the PSA layer (polymer that
shows rubber elasticity in a room temperature range), with the
polymer component(s) accounting for about at least 30% (typically
at least about 40%, e.g. at least about 50%) by weight of the PSA
layer.
(Rubber-Based Polymer)
[0027] The PSA layer disclosed herein is preferably a rubber-based
PSA layer formed from a PSA composition that comprises a
rubber-based polymer as the polymer A. Examples of the rubber-based
polymer include various rubber-based polymers such as natural
rubber; styrene-butadiene rubber (SBR); polyisoprene; a
butene-based polymer comprising butene (referring to 1-butene as
well as cis- or trans-2-butene) and/or 2-methylpropene
(isobutylene) as the primary monomer(s); A-B-A block copolymer
rubber and a hydrogenation product thereof, for instance,
styrene-butadiene-styrene block copolymer rubber (SBS),
styrene-isoprene-styrene block copolymer rubber (SIS),
styrene-isobutylene-styrene block copolymer rubber (SIBS),
styrene-vinyl isoprene-styrene block copolymer rubber (SVIS),
styrene-ethylene-butylene-styrene block copolymer rubber (SEBS)
which is a hydrogenation product of SBS,
styrene-ethylene-propylene-styrene block copolymer rubber (SEPS)
which is a hydrogenation product of SIS, and
styrene-isoprene-propylene-styrene block copolymer (SIPS). Among
these rubber-based polymers, solely one species or a combination of
two or more species can be used.
[0028] A favorable example of the butene-based polymer is an
isobutylene-based polymer. Due to its molecular structure, the
isobutylene-based polymer is highly hydrophobic and its main chain
has low motility. Thus, a PSA layer (isobutylene-based PSA layer)
whose base polymer is an isobutylene-based polymer may itself show
a relatively low moisture permeability. This is also advantageous
from the standpoint of preventing lateral entry of water vapor into
the PSA layer at an edge face of the PSA sheet. Such a PSA layer
tends to have a good elastic modulus and excellent removability.
Specific examples of the isobutylene-based polymer include
polyisobutylene and isobutylene-isoprene copolymer (butyl
rubber).
[0029] The monomers (monomer mixture) to form the rubber-based
polymer disclosed herein comprises one, two or more species of
monomers selected among butene, isobutylene, isoprene, butadiene,
styrene, ethylene and propylene. The rubber-based polymer can be a
polymer obtainable by polymerizing the one, two or more species of
monomers exemplified above. The monomer mixture for forming the
rubber-based polymer disclosed herein typically comprises the one,
two or more species of monomers at a ratio of at least 50% (e.g.
50% to 100%) by weight, preferably at least 75% by weight, more
preferably at least 85% by weight, or yet more preferably at least
90% (e.g. at least 95%) by weight. The ratio of these monomers in
the entire monomer content can also be 99% by weight or higher. The
rubber-based polymer can be obtained by copolymerizing one, two or
more species of other monomers copolymerizable with these monomers
listed as the examples. The rubber-based polymer according to a
preferable embodiment is a polymer obtainable by polymerizing one,
two or more species of monomers selected among isobutylene,
isoprene and butene. It is noted that from the standpoint of
reduction of outgassing (in particular, reduction of gas emission
that may degrade the durability, reliability or accurate operation
of electronic devices including magnetic disc devices), the styrene
content of the monomer mixture is preferably lower than 10% by
weight, or more preferably lower than 1% by weight. The art
disclosed herein can be preferably implemented in an embodiment
where the monomer mixture is essentially free of styrene.
[0030] In a preferable embodiment of the PSA sheet disclosed
herein, the isobutylene-based polymer accounts for more than 50%
(e.g. 70% or more, or even 85% or more) by weight of the base
polyiner(s) in the PSA. The PSA may be essentially free of other
polymers besides the isobutylene-based polymer. In the PSA for
instance, the ratio of non-isobutylene-based polymer content in the
polymer content can be 1% by weight or lower, or at or below the
minimum detectable level.
[0031] As used herein, the "isobutylene-based polymer" is not
limited to isobutylene homopolymer (homopolyisobutylene) and the
term encompasses a copolymer whose primary monomer is isobutylene.
The copolymer includes a copolymer in which isobutylene corresponds
to the highest content of the monomers forming the
isobutylene-based polymer. In typical, it can be a copolymer in
which isobutylene accounts for more than 50% by weight of the
monomers, or even 70% by weight or more thereof. Examples of the
copolymer include a copolymer of isobutylene and butene (normal
butylene), a copolymer (butyl rubber) of isobutylene and isoprene,
vulcanized products and modified products of these. Examples of the
copolymers include butyl rubbers such as regular butyl rubber,
chlorinated butyl rubber, iodinated butyl rubber, and partially
crosslinked butyl rubber. Examples of the vulcanized and modified
products include those modified with functional groups such as
hydroxy group, carboxy group, amino group, and epoxy group. The
isobutylene-based polymer that can be preferably used from the
standpoint of the moisture resistance, reduction of outgassing, and
adhesive strength, etc., includes polyisobutylene and
isobutylene-isoprene copolymer (butyl rubber). The copolymer can be
a copolymer (e.g. an isobutylene-isoprene copolymer) of which the
other monomers (isoprene, etc.) excluding isobutylene has a
copolymerization ratio lower than 30% by mol.
[0032] As used herein, the "polyisobutylene" refers to a
polyisobutylene in which the copolymerization ratio of monomers
excluding isobutylene is 10% or lower (preferably 5% or lower) by
weight.
[0033] For the isobutylene-based polymer, it is possible to
suitably select and use a species having a weight average molecular
weight (Mw) of suitably about 5.times.10.sup.4 or higher,
preferably about 15.times.10.sup.4 or higher, more preferably about
30.times.10.sup.4 or higher, or yet more preferably about
45.times.10.sup.4 or higher (e.g. about 50.times.10.sup.4 or
higher). The maximum Mw is not particularly limited and can be
about 150.times.10.sup.4 or lower (preferably about
100.times.10.sup.4 or lower, e.g. about 80.times.10.sup.4 or
lower). Several species of isobutylene-based polymer varying in Mw
can be used together as well. Having a Mw in these ranges, the PSA
can be easily adjusted to have an elasticity in a preferable range
and is likely to show good cohesive strength.
[0034] While no particular limitations are imposed, as the
polyisobutylene, it is possible to preferably use a species having
a dispersity (Mw/Mn) (which is indicated as a ratio of weight
average molecular weight (Mw) to number average molecular weight
(Mn)) in a range of 3 to 7 (more preferably 3 to 6, e.g. 3.5 to
5.5). Several species of polyisobutylene varying in Mw/Mn can be
used together as well.
[0035] The Mw and Mn values of an isobutylene-based polymer here
refer to values based on standard polystyrene that are determined
by gel permeation chromatography (GPC) analysis. As the GPC
analyzer, for instance, model name HLC-8120 GPC available from
Tosoh Corporation can be used.
[0036] As the butyl rubber, a species with
Mw.gtoreq.5.times.10.sup.4 can be suitably selected and used. In
view of the balance between the PSA layer's ease of formation and
tightness of bonding to adherend (adhesive strength), the butyl
rubber's Mw is preferably about 10.times.10.sup.4 or higher, more
preferably about 15.times.10.sup.4 or higher, yet more preferably
about 30.times.10.sup.4 or higher, or particularly preferably about
45.times.10.sup.4 or higher (e.g. about 50.times.10.sup.4 or
higher); it is preferably 100.times.10.sup.4 or lower, or more
preferably 80.times.10.sup.4 or lower. Several species of butyl
rubber varying in Mw can be used together as well.
[0037] While no particular limitations are imposed, the butyl
rubber has a dispersity (Mw/Mn) in a range of preferably 3 to 8 or
more preferably in a range of 4 to 7. Several species of butyl
rubber varying in Mw/Mn can be used together as well. The butyl
rubber's Mw and Mn can be determined by GPC analysis, similarly to
the polyisobutylene.
[0038] The Mooney viscosity of the butyl rubber is not particularly
limited. For instance, a butyl rubber having a Mooney viscosity
ML.sub.1+8(125.degree. C.) between 10 and 100 can be used. In view
of the balance between the PSA layer's ease of formation and
tightness of bonding to adherend (adhesive strength), a butyl
rubber having a Mooney viscosity ML.sub.1+8(125.degree. C.) of 15
to 80 (more preferably 30 to 70, e.g. 40 to 60) is preferable.
[0039] In a preferable embodiment of the art disclosed herein, the
PSA layer comprises a rubber-based polymer A1 and a rubber-based
polymer A2 as its base polymers. The rubber-based polymers A1 and
A2 are preferably both isobutylene-based polymers. The rubber-based
polymer A1 according to a more preferable embodiment is an
isobutylene-based polymer in which isobutylene is polymerized at a
ratio of at least 50% (e.g. at least 70%, preferably at least 80%,
or yet more preferably at least 90%) by weight; it is typically
polyisobutylene. The rubber-based polymer A2 is an
isobutylene-based polymer in which isobutylene and isoprene are
copolymerized (i.e. an isobutylene-based copolymer); it is
typically au isobutylene-isoprene copolymer. In the copolymer, the
combined amount of isobutylene and isoprene as monomers accounts
for typically at least 50% (e.g. at least 70%, preferably at least
80%, or yet more preferably at least 90%) by weight of the entire
monomers. The use of rubber-based polymers A1 and A2 can bring the
PSA layer's elastic modulus in a preferable range and greater
moisture resistance can be obtained.
[0040] When rubber-based polymers A1 and A2 are used, their blend
ratio can be suitably selected so as to obtain preferable elastic
modulus, moisture resistance and adhesive properties disclosed
herein. The weight ratio (P.sub.A1/P.sub.A2) of rubber-based
polymer A1 (P.sub.A1) to rubber-based polymer A2 (P.sub.A2) can be,
for instance, 95/5 to 5/95, preferably 90/10 to 10/90, more
preferably 80/20 to 20/80, yet more preferably 70/30 to 30/70, or
particularly preferably 60/40 to 40/60.
[0041] In a preferable embodiment, the dispersity (Mw/Mn) of the
aforementioned base polymers at large is 3 or higher, or more
preferably 4 or higher. According to the PSA comprising such base
polymers, adhesive strength can be easily combined with resistance
to leftover adhesive residue. It also brings the PSA layer's
elastic modulus in a favorable range and good moisture resistance
tends to be obtained. At or above a certain Mw/Mn value, the PSA
can be obtained with a low solution viscosity for its Mw. The
dispersity of the base polymers at large can also be 5 or higher, 6
or higher, or even 7 or higher. The maximum dispersity of the base
polymers at large is not particularly limited; it is preferably 10
or lower (e.g. 8 or lower).
[0042] The art disclosed herein can be preferably implemented in an
embodiment having a PSA layer (e.g. a rubber-based PSA layer)
formed of a PSA (a non-crosslinked PSA) in which the based polymers
are not crosslinked. Here, the term "PSA layer formed of a
non-crosslinked PSA" refers to a PSA layer that has not been
subjected to an intentional treatment (i.e. crosslinking treatment,
e.g. addition of a crosslinking agent, etc.) for forming chemical
bonds among the base polymers.
(Acrylic Polymer)
[0043] In an embodiment of the art disclosed herein, the PSA layer
is an acrylic PSA layer comprising an acrylic polymer as the
polymer A. The acrylic polymer is preferably a polymer of a
starting monomer mixture that comprises an alkyl (meth)acrylate as
the primary monomer and may further comprise a secondary monomer
copolymerizable with the primary monomer. Here, the primary monomer
refers to a component accounting for more than 50% by weight of the
starting monomer mixture.
[0044] As used herein, the term "(meth)acryloyl" comprehensively
refers to acryloyl and methacryloyl. Similarly the term
"(meth)acrylate" comprehensively refers to acrylate and
methacrylate, and the term "(meth)acryl" comprehensively refers to
acryl and methacryl.
[0045] As the alkyl (meth)acrylate, for instance, a compound
represented by the following formula (1) can preferably be
used:
CH.sub.2.dbd.C(R.sup.1)COOR.sup.2 (1)
[0046] Here, R.sup.1 in the formula (1) is a hydrogen atom or a
methyl group. R.sup.2 is an acyclic alkyl group having 1 to 20
carbon atoms (hereinafter, such a range of the number of carbon
atoms may be indicated as "C.sub.1-20"). From the standpoint of the
PSA's storage modulus, adhesive properties, etc., an alkyl
(meth)acrylate in which R.sup.2 is a C.sub.1-18 acyclic alkyl group
is preferable; an alkyl (meth)acrylate in which R.sup.2 is a
C.sub.2-14 acyclic alkyl group is more preferable; an alkyl
(meth)acrylate in which R.sup.2 is a C.sub.4-12 acyclic alkyl group
is even more preferable. In particular, it is preferable to use an
alkyl acrylate as the primary monomer. The acyclic alkyl group
includes linear and branched alkyl groups. For the alkyl
(meth)acrylate, solely one species or a combination of two or more
species can be used.
[0047] From the standpoint of the moisture resistance, as the
primary monomer forming the acrylic polymer, it is preferable to
use an alkyl (meth)acrylate having a higher number of carbon atoms
in the acyclic alkyl group. With increasing number of carbon atoms
of side-chain alkyl group in the acrylic polymer, the polymer tends
to have higher hydrophobicity and greater moisture resistance. The
number of carbon atoms in the acyclic alkyl group is 2 or higher,
preferably 4 or higher, more preferably 8 or higher, yet more
preferably 9 or higher, or particularly preferably 12 or
higher.
[0048] The ratio of alkyl (meth)acrylate as the primary monomer in
all the monomers forming the acrylic polymer is preferably 60% by
weight or higher, more preferably 70% by weight or higher, or more
preferably 75% by weight or higher (e.g. 85% by weight or higher).
The maximum alkyl (meth)acrylate content is not particularly
limited; it is preferably 95% by weight or lower (e.g. by weight or
lower).
[0049] Secondary monomers capable of introducing possible
crosslinking points into the acrylic polymer or enhancing the
adhesive strength include hydroxy group-containing monomers (e.g.
2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate,
etc.), carboxy group-containing monomers (e.g. acrylic acid,
methacrylic acid, etc.), acid anhydride group-containing monomers,
amide group-containing monomers, amino group-containing monomers,
imide group-containing monomers, epoxy group-containing monomers,
(meth)acryloylmorpholine, and vinyl ethers. Among them, hydroxy
group-containing monomers and carboxy group-containing monomers are
preferable. Hydroxy group-containing monomers are more preferable.
For the secondary monomer, solely one species or a combination of
two or more species can be used.
[0050] When the monomers forming the acrylic polymer comprises a
functional group-containing monomer, from the standpoint of the
cohesive strength, etc., the ratio of the functional
group-containing monomer in the monomers is suitably 0.1% by weight
or higher, preferably 1% by weight or higher, or more preferably 3%
by weight or higher. The upper limit is preferably 30% by weight or
lower (e.g. 25% by weight or lower).
[0051] As the monomers forming the acrylic polymer, for a purpose
such as increasing the cohesive strength of the acrylic polymer,
other comonomers can be used besides the aforementioned secondary
monomers. Examples of the comonomers include vinyl ester-based
monomers such as vinyl acetate; aromatic vinyl compounds such as
styrene; cycloalkyl (meth)acrylates such as cyclohexyl
(meth)acrylate; aromatic ring-containing (meth)acrylates such as
aryl (meth)acrylates; olefinic monomers such as ethylene,
propylene, isoprene, butadiene and isobutylene; polyfunctional
monomers such as 1,6-hexanediol di(meth)acrylate, having two or
more (e.g. three or more) polymerizable functional groups (e.g.
(meth)acryloyl groups) per molecule. The amount of the other
comonomers can be suitably selected in accordance to the purpose
and application and is not particularly limited. It is preferably
10% by weight or less (e.g. 1% by weight or less) of the
monomers.
[0052] The composition of the monomers forming the acrylic polymer
suitably designed so that the acrylic polymer has a glass
transition temperature (Tg) in a prescribed range.
[0053] Here, the Tg of the acrylic polymer refers to the value
determined by the Fox equation based on the composition of the
monomers. As shown below, the Fox equation is a relational
expression between the Tg of a copolymer and glass transition
temperatures Tgi of homopolymers of the respective monomers
constituting the copolymer.
1/Tg=.SIGMA.(Wi/Tgi)
[0054] In the Fox equation, Tg represents the glass transition
temperature (unit: K) of the copolymer, Wi the weight fraction
(copolymerization ratio by weight) of a monomer i in the copolymer,
and Tgi the glass transition temperature (unit: K) of homopolymer
of the monomer i. As the glass transition temperatures of
homopolymers used for determining the Tg value, values found in
publicly known documents are used. For instance, values given in
"Polymer Handbook" (3rd edition, John Wiley & Sons, Inc., Year
1989) can be used. When the literature provides two or more values
for a certain monomer, the highest value is used.
[0055] While no particular limitations are imposed, from the
standpoint of the adhesion, the acrylic polymer's Tg is
advantageously about 0.degree. C. or lower, or preferably about
-5.degree. C. or lower (e.g. about -15.degree. C. or lower, or
-25.degree. C. or lower). From the standpoint of the PSA layer's
cohesive strength, the acrylic polymer's Tg is about -75.degree. C.
or higher, or preferably about -70.degree. C. or higher (e.g.
-50.degree. C. or higher, or even -30.degree. C. or higher). The
acrylic polymer's Tg can be adjusted by suitably changing the
monomer composition (i.e. the monomer species used for synthesizing
the polymer and their ratio).
[0056] The acrylic polymer's Mw is not particularly limited. For
instance, it can be about 10.times.10.sup.4 or higher and
500.times.10.sup.4 or lower. From the standpoint of the cohesion,
the Mw is about 30.times.10.sup.4 or higher and suitably about
45.times.10.sup.4 or higher (e.g. about 65.times.10.sup.4 or
higher). In a preferable embodiment, the acrylic Mw is
70.times.10.sup.4 or higher, more preferably about
90.times.10.sup.4 or higher, or yet more preferably about
110.times.10.sup.4 or higher. The Mw is suitably 300.times.10.sup.4
or lower (more preferably about 200.times.10.sup.4 or lower, e.g.
about 150.times.10.sup.4 or lower).
[0057] It is noted that Mw is determined from a value obtained
based on standard GPC by GPC. As the analyzer, for instance, model
name HLC-8320 GPC (columns: TSKgel GMH-H(S) available from Tosoh
Corporation) can be used.
[0058] The method for obtaining the acrylic polymer is not
particularly limited. Various polymerization methods known as
synthetic methods of acrylic polymers may be appropriately
employed, such as solution polymerization, emulsion polymerization,
bulk polymerization, suspension polymerization and
photopolymerization. For instance, solution polymerization may be
preferably employed. As the method for supplying the monomers when
solution polymerization is carried out, all-at-once supply by which
all starting monomers are supplied at once, continuous supply
(addition), portion-wise supply (addition) and like method can be
suitably employed. For the solvent (polymerization solvent) used
for solution polymerization, a suitable species can be selected
among heretofore known organic solvents (e.g. toluene and ethyl
acetate). The polymerization temperature can be appropriately
selected according to the species of monomers, solvent, and
polymerization initiator used, etc. It can be, for instance, about
20.degree. C. to 170.degree. C. (typically about 40.degree. C. to
140.degree. C.). In a preferable embodiment, the polymerization
temperature can be about 75.degree. C. or lower (more preferably
about 65.degree. C. or lower, e.g. about 45.degree. C. to
65.degree. C.).
[0059] The initiator used in the polymerization can be suitably
selected among heretofore known polymerization initiators in
accordance with the polymerization method. For instance, one, two
or more species of azo-based polymerization initiator can be
preferably used, such as 2,2'-azobisisobutylonitrile (AIBN). In
radical polymerization, as compared to organic peroxides and other
radial polymerization initiators, it is advantageous to use an
azo-based polymerization initiator as its degradation products are
unlikely to remain in the resulting PSA composition as components
that can be thermally released as gas and outgassing is likely to
be inhibited. Other examples of polymerization initiator include
peroxide-based initiators such as benzoyl peroxide (BPO) and
hydrogen peroxide. Other polymerization initiators include
persulfates such as potassium persulfate; substituted ethane-based
initiators such as phenyl-substituted ethane; aromatic carbonyl
compounds; and redox-based initiators by a combination of a
peroxide and a reducing agent. Among these polymerization
initiators, solely one species or a combination of two or more
species can be used. The polymerization initiator can be used in a
typical amount selected from a range of, for instance, about 0.005
part to 1 part (typically about 0.01 part to 1 part) by weight to
100 parts by weight of the monomers.
(Blend of Acrylic Polymer and Rubber-Based Polymer)
[0060] The PSA layer according to an embodiment of the art
disclosed herein is a rubber-acrylic blend PSA layer comprising a
rubber-based polymer and an acrylic polymer as the polymer A. As
the rubber-based polymer, one, two or more species can be used
among the aforementioned rubber-based polymers. As the acrylic
polymer one, two or more species can be used among the
aforementioned acrylic polymers. The rubber-based polymer and
acrylic polymer can be suitably mixed together to preferably
combine the rubber-based polymers advantage (moisture resistance,
etc.) and acrylic polymers advantage (low level of outgassing,
adhesive properties, etc.). When a rubber-based polymer and an
acrylic polymer are used together as the polymer A, the weight
ratio of rubber-based polymer (R) to acrylic polymer (A), R/A, can
be, for instance, 95/5 to 20/80; it is preferably 90/10 to 30/70,
more preferably 80/20 to 40/60, or yet more preferably 70/30 to
50/50.
(Crosslinking Agent)
[0061] The PSA composition (preferably a solvent-based PSA
composition) used for forming the PSA layer preferably comprises a
crosslinking agent as an optional component. The PSA layer (e.g. an
acrylic PSA layer) in the art disclosed herein may include the
crosslinking agent in a post-crosslinking-reaction form, a
pre-crosslinking-reaction form, a partially-crosslinked form, an
intermediate or combined form of these, etc. In typical, the
crosslinking agent is mostly included in the
post-crosslinking-reaction form.
[0062] The type of crosslinking agent is not particularly limited.
A suitable species can be selected and used among heretofore known
crosslinking agents. Examples of the crosslinking agent, include
isocyanate-based crosslinking agents, epoxy-based crosslinking
agents, oxazoline-based crosslinking agents, aziridine-based
crosslinking agents, melamine-based crosslinking agents,
carbodiimide-based crosslinking agents, hydrazine-based
crosslinking agents, amine-based crosslinking agents,
peroxide-based crosslinking agents, metal chelate-based
crosslinking agents, metal alkoxide-based crosslinking agents, and
metal salt-based crosslinking agents. For the crosslinking agent,
solely one species or a combination of two or more species can be
used. Examples of the crosslinking agent that can be preferably
used in the art disclosed herein include isocyanate-based
crosslinking agents and epoxy-based crosslinking agents. In
particular, isocyanate-based crosslinking agents are more
preferable.
[0063] The crosslinking agent content in the PSA composition
disclosed herein is not particularly limited. From the standpoint
of the cohesion, to 100 parts by weight of the base polymer (e.g.
acrylic polymer), it is suitably about 0.001 part by weight or
more, preferably about 0.002 part by weight or more, more
preferably about 0.005 part by weight or more, or yet more
preferably about 0.01 part by weight or more. From the standpoint
of the adhesive strength and elastic modulus, the crosslinking
agent content in the PSA composition is, to 100 parts by weight of
the base polymer (e.g. acrylic polymer), about 20 parts by weight
or less, suitably about 15 parts by weight or less, or preferably
about 10 parts by weight or less (e.g. about 5 parts by weight or
less).
(Polymer B)
[0064] The art disclosed herein is characterized by the PSA layer
comprising a polymer B in addition to the polymer A. The polymer B
is a polymer different from the polymer A, having a lower molecular
weight than the polymer A. In terms of the molecular weight, Mw is
used for comparison to the polymer A. The polymer B is
characterized by having a Mn of 1000 or higher. This can greatly
prevent moisture permeation in in-plane directions of bonding area
of the PSA layer while maintaining a practical level of holding
power. In addition, the amount of outgassing can be limited as
well. From the standpoint of enhancing the moisture resistance,
maintaining the holding power and reducing outgassing, the Mn is
preferably 2000 or higher, or more preferably 2500 or higher.
[0065] The polymer B's molecular weight is lower than that of the
polymer A, having a Mw typically below 5.times.10.sup.4. From the
standpoint of the moisture resistance, the polymer B's Mw can also
be below about 1.times.10.sup.4, or even about 5000 or lower. The
polymer B according to an embodiment is liquid or a viscous fluid
at room temperature (e.g. 25.degree. C.).
[0066] As the polymer B's Mn, a value determined by vapor pressure
osmometry is used. The polymer B's Mw refers to the value based on
standard polystyrene determined by GPC analysis. As the GPC
analyzer, for instance, model name HLC-8120 GPC available from
Tosoh Corporation can be used.
[0067] The species of polymer B is not particularly limited and a
suitable species is selected in accordance with the species of
polymer A as the base polymer. As the polymer B, one, two or more
species can be used among, for instance, rubber-based polymers
(typically diene-based polymers), olefinic polymers, acrylic
polymers, polyester-based polymers, urethane-based polymers,
polyether-based polymers, silicone-based polymers, polyamide-based
polymers, and fluoropolymers. For use in a magnetic disc device, it
is desirable that the polymer B is essentially free of a
silicone-based polymer that may form siloxane gas.
[0068] The polymer B according to a preferable embodiment is
selected among olefinic polymers and diene-based polymers.
Presumably these polymers are likely to block passage of water
molecules because they generally have low polarity with short side
chains. In addition, it tends to be readily dissolved or dispersed
in the PSA layer when the polymer A is a rubber-based polymer. In
particular, olefinic polymers are more preferable as they are
thermally stable and highly weather resistant. The monomers for
forming the polymer B can be one, two or more species of monomers
selected among ethylene propylene, butene, isobutylene, isoprene
and butadiene. Here, the butene encompasses 1-butene as well as
cis- and trans-2-butenes. The polymer B is preferably a polymer
formed from a monomer mixture that includes one, two or more
species of the monomers exemplified above at a ratio of at least
50% by weight. Specific examples include ethylene-butene copolymer,
ethylene-propylene-butene copolymer, propylene-butene copolymer,
ethylene-butene-unconjugated diene copolymer, and
ethylene-propylene-butene-unconjugated diene copolymer. These
polymers include so-called ethylene propylene rubber.
[0069] In a more preferable embodiment, the polymer B is a polymer
formed from a monomer mixture that includes at least one species of
monomer at a ratio of at least 50% by weight, selected from the
group consisting of butene, isobutylene and isoprene. The polymer
obtained from these monomers is hydrophobic and non-polar; and
therefore, it is likely to bring about excellent moisture
resistance, combined with the effect to prevent moisture permeation
based on the molecular weight in the prescribed range. The monomer
mixture to form the polymer B includes one, two or more species of
the monomers at a ratio of more preferably at least 75% by weight,
yet more preferably at least 85% by weight, or particularly
preferably at least 90% (e.g. at least 95%) by weight. The ratio of
these monomers in the entire monomers can also be 99% by weight or
more. The polymer B may be obtained by copolymerizing one, two or
more species of other monomers (e.g. butadiene, styrene, ethylene,
and propylene) copolymerizable with the monomers exemplified
above.
[0070] In a particularly preferable embodiment, the polymer B is a
polybutene, that is, a polymer formed from a monomer mixture that
includes a monomer selected among butene (1-butene, cis- or
trans-2-butene) and isobutene (isobutylene) at a ratio of at least
50% by weight. The polymerization ratio of butene and isobutene in
the polybutene as the polymer B is preferably about 75% by weight
or higher, more preferably about 85% by weight or higher, or yet
more preferably about 90% by weight or higher (e.g. about 95% by
weight or higher). The butene and isobutene content in the entire
monomers can also be 99% by weight or higher.
[0071] The polybutene is a polymer formed from a monomer mixture
that includes isobutene as the primary component and may
arbitrarily include a certain amount of normal butene (1-butene,
cis- or trans-2-butene). Polybutene is thermally stable and highly
weather resistant because, unlike diene-based rubber, its molecular
chain is free of a double bond. It is also highly moisture
resistant. The primary component is isobutene with a short side
chain (--CH.sub.3) and the motility of the main chain is low
because of the molecular structure; and therefore, it is likely to
block passage of water molecules. The copolymerization ratio of
isobutene in the polybutene is preferably about 50% by weight or
higher, more preferably about 70% by weight or higher, or possibly
about 80% by weight or higher (e.g. about 90% by weight or
higher).
[0072] The polymer B can be obtained by a method suitably selected
from various known polymerization methods. Alternatively a
commercial product corresponding to the polymer B can be obtained
and used. For instance, a polybutene can be obtained by
polymerizing a monomer mixture that includes butene and isobutene
with the use of a Lewis acid catalyst (e.g. aluminum chloride,
boron trifluoride), etc. Alternatively a species corresponding to
the polymer B can be selected and used among commercial products
such as the NISSEKI POLYBUTENE series available from JXTG Nippon
Oil & Energy Corporation and the NICHIYU POLYBUTENE series
available from NOF Corporation.
[0073] In the PSA layer disclosed herein, the ratio
(C.sub.B/C.sub.A) of polymer B content (C.sub.B) to polymer A
content (C.sub.A) is suitably about 0.1 or higher. From the
standpoint of the moisture resistance, the C.sub.B/C.sub.A ratio is
preferably about 0.3 or higher, more preferably about 0.5 or
higher, or yet more preferably about 0.7 or higher (e.g. about 0.9
or higher). The C.sub.B/C.sub.A ratio is suitably about 2 or lower.
From the standpoint of inhibiting a decrease in holding power, the
C.sub.B/C.sub.A ratio is preferably about 1.5 or lower, or more
preferably about 1.2 or lower (e.g. about 1.1 or lower).
[0074] The polymer B content in the layer disclosed herein is
suitably selected in view of the effect of polymer B. From the
standpoint of the moisture resistance, the polymer B content in the
PSA layer is suitably about 10% by weight or higher, preferably
about 20% by weight or higher, more preferably about 30% by weight
or higher, or yet more preferably about 40% by weight or higher.
From the standpoint of inhibiting a decrease in holding power, the
polymer B content in the PSA layer is suitably about 70% by weight
or lower, or preferably about 60% by weight or lower (e.g. about
55% by weight or lower).
[0075] In a preferable embodiment, the PSA layer may have a
composition such at the combined amount of polymer A as a base
polymer and polymer B accounts for more than 50% by weight of the
PSA layer's total weight (i.e. the weight of the PSA layer formed
with the PSA). For instance, the combined amount of polymer A and
polymer B is preferably about 75% by weight or more of the PSA
layer's total weight, more preferably about 85% by weight or more,
or yet more preferably about 90% by weight or more (e.g. 95% by
weight or more).
(Other Additives)
[0076] Besides the components described above, the PSA composition
may comprise, as necessary, various additives generally known in
the field of PSA, such as tackifier (tackifier resin), leveling
agent, crosslinking accelerator, plasticizer, filler, colorant such
as pigment and dye, softener, anti-static agent, anti-aging agent,
UV absorber, antioxidant and photo-stabilizer. It may arbitrarily
include a third polymer that is not either a polymer A or B. With
respect to these various additives, heretofore known species can be
used by typical methods. From the standpoint of the accuracy of
reading and writing by the magnetic disc device, the art disclosed
herein can be preferably implemented in an embodiment where the PSA
layer has a composition essentially free of, for instance, UV
absorber, antioxidant, and photo-stabilizer such as hindered
amine-based photo-stabilizer and hindered phenolic antioxidant.
[0077] Some applications may allow the PSA sheet disclosed herein
to outgas only up to a certain limit. Thus, it is desirable to
avoid the use of a low molecular weight component that may lead to
outgassing. From such a standpoint, the additive content (e.g.
tackifier resin, anti-aging agent, UV absorber, antioxidant,
photo-stabilizer) in the PSA layer is preferably limited to below
about 30% (e.g. below 10% typically below 3%) by weight. The art
disclosed herein can be preferably implemented in an embodiment
where the PSA layer is essentially free of other additives (e.g.
tackifier resin, anti-aging agent, UV absorber, antioxidant, and
photo-stabilizer).
[0078] The PSA layer can be formed based on a method for forming a
PSA layer in a known PSA sheet. For example, it is preferable to
use a method (direct method) where a PSA composition having
PSA-layer forming materials dissolved or dispersed in a suitable
solvent is directly provided (typically applied) to a substrate (a
moisture-impermeable layer) and allowed to dry to form a PSA layer.
In another method (transfer method) that can be employed, the PSA
composition is provided to a highly-releasable surface (e.g. a
surface of a release liner, a substrate's back face that has been
treated with release agent, etc.) and allowed to dry to form a PSA
layer on the surface, and the PSA layer is transferred to a support
substrate (a moisture-impermeable layer). As the release face, a
surface of a release liner, a substrate's back face that has been
treated with release agent, and the like can be used. The PSA layer
disclosed herein is typically formed in a continuous manner.
[0079] The form of the PSA composition is not particularly limited.
For instance, it can be in various forms, such as a PSA composition
(a solvent-based PSA composition) that comprises PSA-layer-forming
materials as described above in an organic solvent, a PSA
composition (water-dispersed PSA composition, typically an aqueous
emulsion-based PSA composition) in which the PSA is dispersed in an
aqueous solvent, a PSA composition that is curable by an active
energy ray (e.g. UV ray), and a hot-melt PSA composition. From the
standpoint of the ease of application and the adhesive properties,
a solvent-based PSA composition can be preferably used. As the
solvent, it is possible to use one species of solvent or a mixture
of two or more species, selected among aromatic compounds
(typically aromatic hydrocarbons) such as toluene and xylene;
acetic acid esters such as ethyl acetate and butyl acetate; and
aliphatic or alicyclic hydrocarbons such as hexane, cyclohexane,
heptane and methyl cyclohexane. While no particular limitations are
imposed, it is usually suitable to adjust the solvent-based PSA
composition to include 5% to 30% non-volatiles (NV) by weight. Too
low an NV tends to result in higher production costs while too high
an NV may degrade the handling properties such as the ease of
application.
[0080] The PSA composition can be applied, for instance, with a
known or commonly used coater such as gravure roll coater, reverse
roll coater, kiss roll coater, dip roll coater, bar coater knife
coater, and spray coater.
[0081] In the art disclosed herein, the thickness of the PSA layer
forming the adhesive face is not particularly limited. The PSA
layer has a thickness of suitably 3 .mu.m or greater, preferably 10
.mu.m or greater, or more preferably 20 .mu.m or greater. With
increasing thickness of the PSA layer, the adhesive strength to
adherend tends to increase. Having at least a certain thickness,
the PSA layer absorbs the adherend's surface roughness to form
tight adhesion. When the PSA layer has a thickness of 10 .mu.m or
greater, for instance, it can provide good, tight adhesion to an
adherend having a surface whose arithmetic mean surface roughness
Ra is about 1 .mu.m to 5 .mu.m (e.g. 3 .mu.m). The thickness of the
PSA layer forming the adhesive face can be, for instance, 150 .mu.m
or less; it is suitably 100 .mu.m or less, or preferably 50 .mu.m
or less. With decreasing thickness of the PSA layer, it tends to
show a greater ability to inhibit water vapor from laterally
permeating the PSA layer, leading to reduction of outgassing from
the PSA layer. A smaller thickness of the PSA layer is also
advantageous from the standpoint of reducing the thickness and
weight of the PSA sheet.
(Properties of PSA Layer)
[0082] The storage modulus at 25.degree. C., G'(25.degree. C.), of
the PSA layer disclosed herein is not particularly limited and it
can be set in a suitable range according to required properties,
etc. In a preferable embodiment, the G'(25.degree. C.) is less than
0.5 MPa. The PSA layer with G'(25.degree. C.) at or below a
prescribed value wets the adherend surface well to form tight
adhesion. The G'(25.degree. C.) is more preferably 0.4 MPa or less,
yet more preferably 0.3 MPa or less, or particularly preferably
0.25 MPa or less. The G'(25.degree. C.) can also be, for instance,
0.2 MPa or less). The G'(25.degree. C.) value is not particularly
limited and is suitably greater than about 0.01 MPa. From the
standpoint of the adhesive properties and of preventing leftover
adhesive residue, etc., it is preferably 0.05 MPa or greater, or
more preferably 0.07 MPa or greater (e.g. 0.1 MPa or greater).
[0083] In the art disclosed herein, the storage moduli
G'(25.degree. C.) of a PSA layer can be determined by dynamic
elastic modulus measurement. In particular, several layers of the
PSA subject to measurement are layered to fabricate an
approximately 2 mm thick PSA layer. A specimen obtained by punching
out a disc of 7.9 mm diameter from the PSA layer is fixed between
parallel plates. With a rheometer (e.g. ARES available from TA
Instruments or a comparable system), dynamic elastic modulus
measurement is carried out to determine the storage moduli
G'(25.degree. C.). The PSA (layer) subject to measurement can be
formed by applying a layer the corresponding PSA composition on a
release face of a release liner or the like and allowing it to dry
or cure. The thickness (coating thickness) of the PSA layer
subjected to the measurement is not particularly limited as long as
it is 2 mm or less. It can be, for instance, about 50 .mu.m. [0084]
Measurement mode: shear mode [0085] Temperature range: -50.degree.
C. to 150.degree. C. [0086] Heating rate: 5.degree. C./min [0087]
Measurement frequency: 1 Hz
[0088] The same measurement method is also used in the working
examples described later.
<Moisture-Impermeable Layer>
[0089] As used herein, the moisture-impermeable layer refers to a
layer (film) having a moisture permeability (a water vapor
transmission rate in the thickness direction) lower than
5.times.10.sup.-1 g/m.sup.224 h when determined at 40.degree. C. at
90% RH based on the MOCON method (JIS K7129:2008). The
moisture-impermeable layer in the art disclosed herein is formed by
suitably selecting materials and a way of layering so as to satisfy
the moisture impermeability. With the use of the
moisture-impermeable layer, it is possible to obtain a PSA sheet
resistant to moisture in the thickness direction. The moisture
permeability is preferably lower than 5.times.10.sup.-2 g/m.sup.224
h, or more preferably lower than 5.times.10.sup.-3 g/m.sup.224 h,
for instance, lower than 5.times.10.sup.-5 g/m.sup.224 h. As the
moisture permeability tester, PERMATRAN W3/33 available from MOCON,
Inc. or a comparable product can be used. It is noted that in the
PSA sheet disclosed herein, the moisture-impermeable layer can also
serve as a substrate (support substrate) to support the PSA
layer.
[0090] In a preferable embodiment, the moisture-impermeable layer
disclosed herein includes an inorganic layer. The material or
structure of the inorganic layer is not particularly limited and
can be selected in accordance of the purpose and usage. From the
standpoint of the moisture resistance and airtight properties, it
is advantageous that the inorganic layer is essentially non-porous.
In typical a preferable inorganic layer is essentially formed of an
inorganic material. For instance, an inorganic layer formed of at
least 95% (by weight) inorganic material is preferable (more
preferably at least 98% by weight, or yet more preferably at least
99% by weight). The number of inorganic layers in the
moisture-impermeable layer is not particularly limited; it can be
one, two or more (e.g. about two to five). From the standpoint of
the ease of manufacturing and availability the number of inorganic
layers in the moisture-impermeable layer is preferably about 1 to
3, or more preferably one or two. When the moisture-impermeable
layer includes several inorganic layers, the materials and
structures (thicknesses, etc.) of these inorganic layers can be the
same with or different from one another.
[0091] As the inorganic material forming the inorganic layer, it is
possible to use, for instance, metal materials including elemental
metals such as aluminum, copper, silver, iron, tin, nickel, cobalt,
and chromium as well as alloys of these; and inorganic compounds
such as oxides, nitrides and fluorides of metals and metalloids
including silicon, aluminum, titanium, zirconium, tin and
magnesium. Specific examples of the inorganic compounds include
silicon oxides (SiO.sub.x, typically SiO.sub.2), aluminum oxide
(Al.sub.2O.sub.3), silicon nitride (Si.sub.3N.sub.4), silicon oxide
nitride (SiO.sub.xN.sub.y), titanium oxide (TiO.sub.2), and indium
tin oxide (ITO).
[0092] The metal materials can be used as the inorganic layers as
metal foils (e.g. aluminum foil) formed by a known method such as
rolling by a rolling mill, etc. Alternatively, for instance, a
metal material formed in a layer by a known film-forming method
such as vacuum vapor deposition, spattering and plating.
[0093] The inorganic compound can be typically used as the
inorganic layer in a form of thin film formed by a known method. As
the method for forming thin film of the inorganic compound, various
vapor deposition methods can be used. For instance, physical vapor
deposition methods (PVD) such as vacuum vapor deposition,
spattering and ion plating, chemical vapor deposition methods (CVD)
and like method can be used. The moisture-impermeable layer may
further have a resin layer on top of the vapor deposition layer.
For instance, the resin layer may be a topcoat layer provided for
purposes such as protecting the vapor deposition layer.
[0094] From the standpoint of the moisture resistance, ease of
manufacturing, availability etc., it is preferable to use an
inorganic layer formed of for instance, aluminum or an aluminum
alloy. From the standpoint of the moisture resistance, ease of
manufacturing, availability; etc., as the inorganic layer thrilled
of an inorganic compound, for instance, a silicon oxide layer or an
aluminum oxide layer can be preferably used. Examples of an
inorganic layer preferable for being able to form a highly
transparent, inorganic layer include a silicon oxide layer, an
aluminum oxide layer and an ITO layer.
[0095] The maximum thickness of the inorganic layer is not
particularly limited. From the standpoint of obtaining
conformability to shapes of adherends, the inorganic layer
advantageously has a thickness of 50 .mu.m or less. From the
standpoint of reducing the thickness and weight of the PSA sheet,
the thickness of the inorganic layer is suitably 15 .mu.m or less,
preferably 13 .mu.m or less, more preferably 11 .mu.m or less, or
yet more preferably 9 .mu.m or less. When the moisture-impermeable
layer includes several inorganic layers, the combined thickness of
these inorganic layers is in these ranges. The minimum thickness of
the inorganic layer is not particularly limited and can be suitably
selected so as to obtain a PSA sheet that shows moisture resistance
suited for the purpose and usage. The thickness of the inorganic
layer is suitably 1 nm or greater. From the standpoint of the
moisture resistance, air tight properties, etc., it is preferably 2
nm or greater, or more preferably 5 nm or greater. When the
moisture-impermeable layer includes several inorganic layers, it is
preferable that at least one of them has a thickness in these
ranges. Each of the several inorganic layers may have a thickness
in these ranges as well.
[0096] The preferable thickness range of the inorganic layer may
also vary depending on the material of the inorganic layer, the
formation method, etc. For instance, when metal foil (e.g. aluminum
foil) forms the inorganic layer (or the metal layer), in view of
the moisture resistance, ease of manufacturing, crease resistance,
etc., its thickness is suitably 1 .mu.m or greater, preferably 2
.mu.m or greater, or more preferably 5 .mu.m or greater. In view of
the flexibility which leads to adherend conformability the metal
layer's thickness is suitably 50 .mu.m or less, preferably 20 .mu.m
or less, more preferably 15 .mu.m or less, yet more preferably 12
.mu.m or less, or particularly preferably 10 .mu.m or less. With
respect to the inorganic layer formed by vapor deposition of an
inorganic compound, in view of the balance between flexibility and
ease of manufacturing the moisture-impermeable layer, its thickness
is suitably in a range between 1 nm and 1000 nm, preferably in a
range between 2 nm and 300 nm, or more preferably in a range
between 5 nm and less than 100 nm.
[0097] The moisture-impermeable layer disclosed herein may include
a resin layer in addition to the inorganic layer. The resin layer
may serve as a protection layer to prevent the inorganic layer from
getting damaged by bending deformation and friction. Thus, the
moisture-impermeable layer including the resin layer in addition to
the inorganic layer is preferable from the standpoint of the
endurance and reliability of moisture-resistant properties and also
from the standpoint of the ease of handling the
moisture-impermeable layer or the PSA sheet. By placing the resin
layer on the PSA layer side surface of the moisture impermeable
layer, the anchoring of the PSA layer can be enhanced. When the
inorganic layer is formed by vapor deposition, spattering or like
method, the resin layer can be used as the base for forming the
inorganic layer.
[0098] The structure of the resin layer not particularly limited.
For instance, the resin layer may include a void space as in fiber
assemblies such as woven fabrics and non-woven fabrics or as in
foam bodies such as polyethylene foam; or it can be a resin layer
(resin film) essentially free of a void space. From the standpoint
of reducing the thickness of the PSA sheet, it is preferable use a
resin layer essentially free of a void space.
[0099] As the resin material forming the resin layer, it is
possible to use, for instance, polyester resins such as
polyethylene terephthalate (PET), polybutylene terephthalate (PBT)
and polyethylene naphthalate (PEN); polyolefin resins such as
polyethylene (PE) and polypropylene (PP); polyimide (PI);
polyetheretherketone (PEEK); chlorine-containing polymers such as
polyvinyl chloride (PVC) and polyvinylidene chloride;
polyamide-based resins such as nylon and aramid polyurethane resin
polystyrene-based resin; acrylic resins; fluororesins;
cellulose-based resins; and polycarbonate-based resins. Of these,
solely one species or a combination of two or more species can be
used. When two or more species of resin are used together, these
resins can be used blended or separately Both thermoplastic resins
and thermosetting resins can be used. From the standpoint of the
ease of forming film, etc., a thermoplastic resin is preferably
used.
[0100] In the moisture-impermeable layer including a resin layer,
at an edge face of the PSA sheet, water vapor may enter the resin
layer from its side (lateral surface). From the standpoint of
inhibiting such entrance of water vapor, as the resin material
forming the resin layer, a highly moisture-resistant material can
be preferably used. For instance, a preferable resin layer is
formed, using a resin material whose primary component is a
polyester resin such as PET or a polyolefinic resin such as PE and
PP. In a preferable embodiment, PET film can be preferably used as
the resin layer. In another preferable embodiment, as the resin
layer, it is preferable to use BOPP (biaxially oriented
polypropylene) film obtainable by forming film of a resin material
that comprises PP as the primary component and biaxially stretching
the film. In the PSA sheet having no inorganic layer further on the
adherend side relative to the resin layer, it is particularly
significant to inhibit entrance of water vapor from the lateral
surface of the resin layer. A typical example of the PSA sheet
having such a constitution is a PSA sheet in which the PSA layer
side surface of the moisture-impermeable layer is formed with a
resin layer.
[0101] The resin layer may include, as necessary; various additives
such as fillers (inorganic fillers, organic fillers, etc.),
anti-aging agent, antioxidant, UV absorber, anti-static agent, slip
agent and plasticizer. The ratio of the various additives included
is below about 30% by weight (e.g. below 20% by weight, typically
below 10% by weight).
[0102] The number of resin layers in the moisture-impermeable layer
is not particularly limited and it can be one, two or more (e.g.
about, two to five). From the standpoint of the ease of
manufacturing and availability the number of resin layers in the
moisture-impermeable layer is preferably one to three, or more
preferably one or two. When the moisture-impermeable layer includes
several resin layers, the materials and structures (thicknesses,
inclusion of a void space, etc.) of these resin layers can be the
same with or different from one another.
[0103] The method for forming the resin layer is not particularly
limited. A heretofore known general resin film molding method can
be suitably employed to form the resin layer, for instance,
extrusion molding, inflation molding, T-die casting, calender roll
molding and wet casting. The resin layer may a non stretched kind
or may be subjected to a stretching process such as uni-axial
stretching and biaxial stretching.
[0104] The minimum thickness of the resin layer is not particularly
limited. From the standpoint of the crease resistance, ease of
forming film, etc., the thickness of the resin layer is suitably 1
.mu.m or greater, preferably 3 .mu.m or greater, more preferably 5
.mu.m or greater, or yet more preferably 7 .mu.m or greater. When
the moisture-impermeable layer includes several resin layers, it is
preferable that at least one of them has a thickness in these
ranges. Each of the several resin layers may have a thickness in
these ranges as well.
[0105] The maximum thickness of the resin layer is not particularly
limited. For instance, it can be 100 .mu.m or less. From the
standpoint of reducing the thickness and weight of the PSA sheet,
the thickness of the resin layer is suitably 70 .mu.m or less,
preferably 55 .mu.m or less, or more preferably 35 .mu.m or less.
When the moisture-impermeable layer includes several resin layers;
the combined thickness of these resin layers is preferably in these
ranges. In general, the moisture permeability of the resin layer is
higher than that of the inorganic layer. Thus, it is also
preferable to make the combined thickness of resin layers smaller
from the standpoint of preventing water vapor from entering the
resin layer from its lateral surface.
[0106] The inorganic layer and the resin layer are preferably
bonded. The bonding method is not, particularly limited. A method
known in the pertinent field can be suitably employed. For
instance, it is possible to employ a method (extrusion lamination)
where a resin material for forming the resin layer is melted and
extruded along with a pre-molded inorganic layer (typically metal
foil), a method where a solution or dispersion of the resin
material for forming the resin layer is applied to a pre-molded
inorganic layer and allowed to dry, and like method. Alternatively,
it is also possible to employ a method where an inorganic layer is
vapor-deposited on a pre-molded resin layer, a method where an
inorganic layer is bonded to a separately-molded resin layer, and
like method. For instance, the bonding can be achieved by hot
pressing. The resin lacer and the inorganic layer can be bonded via
an adhesive layer or a PSA layer.
[0107] The adhesive layer to bond the resin layer and the inorganic
layer can be an undercoat layer formed by applying an undercoat
such as primer to the resin layer. As the undercoat, those known in
the pertinent field can be used, such as urethane-based undercoat,
ester-based undercoat, acrylic undercoat, and isocyanate-based
undercoat. From the standpoint of reducing the thickness and weight
of the PSA sheet, the thickness of the undercoat layer is suitably
7 .mu.m or less, preferably 5 .mu.m or less, or more preferably 3
.mu.m or less. The minimum thickness of the undercoat layer is not
particularly limited. For instance, it can be 0.01 .mu.m or greater
(typically 0.1 .mu.m or greater).
[0108] Before the bonding process, the resin layer may be subjected
to common surface treatment, chemical or physical treatment, the
instance, mattifying treatment, corona discharge treatment,
crosslinking treatment, chromic acid treatment, ozone exposure,
flame exposure, high-voltage electric shock exposure, and ionized
radiation treatment.
[0109] The PSA layer(s) placed between layers forming the
moisture-impermeable layer to bond them together are not exposed to
the surface of the PSA sheet; and therefore, they do not correspond
to the PSA layer forming the adhesive thee of the PSA sheet. In the
PSA sheet disclosed herein, the material and physical properties of
such a PSA layer for internal use in the moisture-impermeable layer
are not particularly limited. The PSA layer can be formed of a PSA
similar to the PSA layer forming the adhesive face or can be formed
of a different PSA. It is not particularly limited in thickness,
either. For instance, it may have a comparable thickness to the
undercoat layer.
[0110] Favorable examples of the moisture-impermeable layer used in
the PSA sheet disclosed herein include a moisture-impermeable layer
formed of a laminate body that comprises an inorganic layer as well
as first and second resin layers laminated on top and bottom of the
inorganic layer. The first and second resin layers forming the
moisture-impermeable layer are laminated on top and bottom of the
inorganic layer. As long as such a layer order can be obtained, the
first and second resin layers may be in direct contact with the
inorganic layer or they may be placed via undercoat layers as
described above to obtain tight adhesion between themselves and the
inorganic layer. In the PSA sheet disclosed herein, the first resin
layer refers to the resin layer placed on the backside (the front
face of the moisture-impermeable layer) of the PSA sheet relative
to the inorganic layer and the second resin layer refers to the
resin layer placed on the PSA layer side.
[0111] The inorganic layer can be a metal layer formed of an
aforementioned metal material. For instance, an aluminum layer is
preferable. The first and second resin layers are preferably formed
from the same material. For instance, thermoplastic resins
exemplified above can be used. Of these materials, solely one
species or a combination of two or more species can be used. Each
of the first and second resin layers may have a layered structure
with two or more layers, but is preferably a monolayer. In
particular preferable materials forming the first and second resin
layers include PET, PP and polystyrene. PET and PP are more
preferable.
[0112] The first and second resin layers have thicknesses T.sub.R1
and T.sub.R2, respectively, and their ratio (T.sub.R1/T.sub.R2) is
not particularly limited, but is suitably 0.5 or greater,
preferably 1 or greater, more preferably 1.5 or greater, or yet
more preferably 2.0 or greater. The T.sub.R1/T.sub.R2 ratio is
suitably about 10 or less, preferably 7.0 or less, more preferably
5.0 or less, or yet more preferably 4.0 or less. When the
T.sub.R1/T.sub.R2 ratio is in these ranges, adherend conformability
and crease resistance can be preferably combined. The thickness
T.sub.R1 of the first resin layer is suitably about 10 .mu.m or
greater, preferably 15 .mu.m or greater, more preferably 18 .mu.m
or greater, or yet more preferably 20 .mu.m or greater (e.g. 22
.mu.m or greater). T.sub.R1 is suitably about 100 .mu.m or less,
preferably 70 .mu.m or less, more preferably 60 .mu.m or less, yet
more preferably 50 .mu.m or less, or particularly preferably 35
.mu.m or less. The thickness T.sub.R2 of the second resin layer is
suitably about 1 .mu.m or greater, preferably 3 .mu.m or greater,
more preferably 5 .mu.m or greater, or yet more preferably 7 .mu.m
or greater. T.sub.R2 is suitably about 25 .mu.m or less, preferably
20 .mu.m or less, more preferably 15 .mu.m or less, or yet more
preferably 12 .mu.m or less (e.g. 10 .mu.m or less).
[0113] The inorganic layer has a thickness and the first and second
resin layers have a combined thickness T.sub.R
(=T.sub.R1+T.sub.R2); and their ratio (T.sub.R/T.sub.I) is not
particularly limited. From the standpoint of preventing creases,
protecting the inorganic layer etc., the ratio is suitably 1 or
greater, preferably 2 or greater, more preferably 3 or greater, or
yet more preferably 4 or greater. When it is bent and applied to
accommodate the adherend shape, in view of the adherend
conformability; the T.sub.R/T.sub.I ratio is suitably 10 or less,
preferably 8 or less, or more preferably 6 or less. The total
(T.sub.R) of the first and second resin layers' thicknesses
T.sub.R1 and T.sub.R2 is suitably about 15 .mu.m or greater,
preferably 20 .mu.m or greater, more preferably 25 .mu.m or
greater, or yet more preferably 30 .mu.m or greater. T.sub.R is
suitably about 100 .mu.m or less, preferably 80 .mu.m or less, more
preferably 70 .mu.m or less, or yet more preferably 60 .mu.m or
less (e.g. 50 .mu.m or less). The moisture-impermeable layer in
this embodiment can effectively protect the inorganic layer (e.g.
an aluminum layer) as thin film from bending, creasing, breaking,
etc. By this, even when the PSA sheet is exposed to various
stressors in the manufacturing process, etc., or even when it is
exposed to a harsh environment for a long period while in use, it
can securely maintain the properties as the moisture-resistant
film.
[0114] As the method for forming a laminate body having the
inorganic layer, first resin layer and second resin layer, it is
possible to employ various methods as described earlier, such as a
method where the respective layers are formed as films by a known
method and they are laminated dry by forming undercoat layers
described above, a method where the inorganic layer is formed on
the first resin layer in a tightly bonded manner and the second
resin layer is laminated dry or extrusion-laminated on top of it,
and like method.
[0115] The minimum thickness of the moisture-impermeable layer is
not particularly limited. From the standpoint of the ease of
manufacturing and handling the PSA sheet, the thickness of the
moisture-impermeable layer is about 3 .mu.m or greater, or suitably
about 5 .mu.m or greater (e.g. 10 .mu.m or greater). To obtain
moisture resistance and rigidity unsusceptible to creasing, it is
desirable that the moisture-impermeable layer is thick. From such a
standpoint, the thickness of the moisture-impermeable layer is
preferably 15 .mu.m or greater, more preferably 20 .mu.m or
greater, yet more preferably 30 .mu.m or greater, or particularly
preferably 40 .mu.m or greater. The maximum thickness of the
moisture-impermeable layer is not particularly limited, either. It
is about 1 mm or less, or suitably about 300 .mu.m or less (e.g.
150 .mu.m or less). From the standpoint of the adherend
conformability of the PSA sheet and of reducing its thickness and
weight, the thickness of the moisture-impermeable layer is
preferably 100 .mu.m or less, more preferably 80 .mu.m or less, yet
more preferably 70 .mu.m or less, or particularly preferably 65
.mu.m or less (e.g. 55 .mu.m or less). The moisture-impermeable
layer with such a limited thickness is less likely to lead to
limitation of a space between the adherend and the PSA sheet; and
therefore, it can prevent water vapor permeation through the
space.
[0116] The PSA layer-side surface of the moisture-impermeable layer
may be subjected to common surface treatment, chemical or physical
treatment, for instance, mattifying treatment, corona discharge
treatment, crosslinking treatment, chromic acid treatment, ozone
exposure, flame exposure, high-voltage electric shock exposure, and
ionized radiation treatment. On the PSA layer-side surface of the
moisture-impermeable layer, an undercoat layer may be placed, which
is formed by applying an undercoat such as primer to the resin
layer. As the undercoat, those known in the pertinent field can be
used, such as urethane-based, ester-based, acrylic, and
isocyanate-based kinds. From the standpoint of reducing the
thickness and weight of the PSA sheet, the thickness of the
undercoat layer is suitably 7 .mu.m or less, preferably 5 .mu.m or
less, or more preferably 3 .mu.m or less.
<Total Thickness of PSA Sheet>
[0117] The total thickness of the PSA sheet disclosed herein is not
particularly limited. It is suitably about 6 .mu.m or greater. From
the standpoint of the moisture resistance and crease resistance,
etc., it is preferably 25 .mu.m or greater, more preferably 40
.mu.m or greater, or yet more preferably 60 .mu.m or greater. The
total thickness is suitably about 1.2 mm or less. From the
standpoint of the adherend conformability and of reducing the
thickness and weight, it is preferably 200 .mu.m or less, more
preferably 150 .mu.m or less, or yet more preferably 120 .mu.m or
less (e.g. less than 100 .mu.m). The total thickness of a PSA sheet
here refers to the combined thickness of the moisture-impermeable
layer and the PSA layer, not including the thickness of the release
liner described later.
<Release Liner>
[0118] In the art disclosed herein, a release liner be used during
formation of the PSA layer; fabrication of the PSA sheet; storage,
distribution and shape machining of the PSA sheet, prior to use,
etc. The release liner is not particularly limited. For example, a
release liner having a release layer on the surface of a liner
substrate such as resin film and paper; a release liner formed from
a low adhesive material such as a fluoropolymer
(polytetrafluoroethylene, etc.) or a polyolefinic resin (PE, PP,
etc.); or the like can be used. The release layer can be filmed,
for instance, by subjecting the liner substrate to a surface
treatment with a release agent such as a silicone-based, long-chain
alkyl-based, fluorine-based, or molybdenum disulfide-based release
agent. When the PSA sheet is used as a sealing material for a
magnetic disc device, it is preferable to use a non-silicone-based
release liner free of a silicone-based release agent which may
produce siloxane gas.
<Properties of PSA Sheet>
[0119] The PSA sheet disclosed herein preferably has a
through-bonding-plane moisture permeability lower than 90
.mu.g/cm.sup.224 h when determined at a permeation distance of 2.5
mm based on the MOCON method (equal-pressure method). This limits
the moisture permeation in in-plane directions of the bonding area
(in directions perpendicular to the thickness direction of the PSA
sheet) and excellent moisture resistance can be preferably
obtained. The through-bonding-plane moisture permeability is more
preferably less than 30 .mu.g/cm.sup.224 h, yet more preferably
less than 25 .mu.g/cm.sup.224 h, or particularly preferably less
than 20 .mu.g/cm.sup.224 h.
[0120] In particular, the through-bonding-plane moisture
permeability is determined by the method described below. [0121]
(1) A metal plate having a 50 mm square opening at the center is
obtained. FIG. 2 outlines a moisture permeability tester 50 used
for determining the moisture permeability. In FIG. 2, reference
number 56 shows the metal plate and reference number 58 shows the
opening made in metal plate 56. FIG. 3 shows a top view of metal
plate 56 having opening 58. [0122] (2) The PSA sheet subject to
measurement is cut to a 55 mm square and applied to cover the
opening in the metal plate to prepare a measurement sample. The PSA
sheet is applied to the metal plate over a bonded width of 2.5 mm
at each side of the opening. The PSA sheet is applied by rolling a
2 kg roller back and forth once. The bonded width of the PSA sheet
at each side of the opening is the width of the band of bonding
area between the PSA sheet and the metal plate, indicating the
permeation distance (mm) in the in-plane direction of bonding area
of the PSA sheet. The circumference of the opening in the metal
plate is referred to as the bonded length (mm). The bonded length
(mm) is the total length of the band of bonding area exposed to
water vapor. In particular, the measurement sample has a structure
shown by reference number 60, formed of metal plate 56 and PSA
sheet 1 applied to metal plate 56. [0123] (3) Based on Method B of
JIS K 7129:2008, the measurement sample is placed between a dry
chamber and a wet chamber in the moisture permeability tester. In
particular, as shown in FIG. 2, a measurement sample 60 is
positioned between a dry chamber 54 and a wet chamber 52. In FIG.
2, WV represents water vapor. [0124] (4) Based on the MOCON method
(equal-pressure method), conditioning is carried out for 3 hours.
Subsequently, as shown in FIG. 2, at 40.degree. C. and 90% RH, the
amount (.mu.g) of moisture that has permeated in in-plane
directions of bonding area of PSA sheet per 6 hours is determined.
[0125] (5) To obtain the through-bonding-plane moisture
permeability (.mu.g/cm.sup.224 h), the amount of permeated moisture
per 24 hours converted from the measurement value and the PSA
layer's surface area (permeation distance.times.bonded length) are
substituted into the equation:
[0125] Moisture permeability (.mu.g/cm.sup.224 h)=amount of
permeated moisture (.mu.g)/(permeation distance (cm).times.bonded
length (cm).times.24 h)
[0126] As used herein, the "through-bonding-plane moisture
permeability (.mu.g/cm.sup.224 h) determined at a permeation
distance of 2.5 mm based on the MOCON method (equal-pressure
method)" can be a value obtained by a measurement of 24 hours, but
it is not limited to this; as described above, a 24-hour value
converted from a measurement taken for a certain time period (e.g.
6 hours) can be used. A 24-hour value converted from a measurement
taken for a time period longer than 6 hours can be used as
well.
[0127] The kind of metal plate is not particularly limited. For
instance, an aluminum plate can be used. The size of the metal
plate is not particularly limited, either. In accordance with the
tester size, etc., for instance, a 100 mm square plate can be used.
It is suitable to use a metal plate having a smooth surface, for
instance, one having a mean arithmetic roughness Ra of about 3
.mu.m or less. As the tester, product name PERMATRAN-3/34G
available from MOCON, Inc. or a comparable product can be used. In
a tester of this type, N.sub.2 gas at 90% RH can be supplied to the
wet chamber and N.sub.2 gas at 0% RH can be supplied to the dry
chamber. This maintains the two chambers divided by the measurement
sample at an equal pressure. In the tester, the water vapor
concentration is measured by an infrared sensor (indicated as "IR"
in FIG. 2), but the means of detection is not limited to this. The
position of the measurement sample in the tester is not
particularly limited. The adhesive face of the PSA sheet can be
placed either on the wet chamber side or on the dry chamber side.
The same measurement method is employed in the working examples
described later.
[0128] This measurement method has been created by the present
inventors. This method can accurately measure the amount of
moisture that permeates in the in-plane direction, which has been
heretofore impossible. More specifically, between different samples
showing similar values in the moisture permeability test by the
conventional cup method, a significant difference in
through-bonding-plane moisture permeability can be detected. The
use of this method allows for a higher-level evaluation of moisture
resistance. For instance, it can quantify a minute water vapor
transmission which may affect HAMR.
[0129] The PSA sheet disclosed herein preferably has an amount of
thermally released gas of 10 .mu.g/cm.sup.2 or less (in particular,
0 to 10 .mu.g/cm.sup.2) when determined at 130.degree. C. for 30
minutes by GC-MS. The PSA sheet with such highly-limited thermal as
release can be preferably used in an application (typically a
magnetic disc device) for which the presence of volatile gas is
undesirable. When the PSA sheet satisfying this property is used as
a sealing material for a magnetic disc device, it can highly
inhibit internal contamination with siloxane and other gas that
affect the device. The amount of thermally released gas is
preferably 7 .mu.g/cm.sup.2 or less, more preferably 5
.mu.g/cm.sup.2 or less, yet more preferably 3 .mu.g/cm.sup.2 or
less, or particularly preferably 1 .mu.g/cm.sup.2 or less.
[0130] The amount of thermally released gas is determined based on
the dynamic headspace method. In particular, a PSA sheet subject to
measurement is cut out to a 7 cm.sup.2 size to obtain a measurement
sample. The measurement sample is sealed in a 50 mL vial and heated
at 130.degree. C. for 30 minutes, using a headspace autosampler. As
the headspace autosampler, a commercial product can be used without
particular limitations. For instance, product name EQ-12031HSA
available from JEOL Ltd., or a comparable product can be used. The
total amount of gas released from the measurement sample is
determined by gas chromatography/mass spectrometry (GC-MS). A
commercial CC-MS can be used. The amount of thermally released gas
is the amount of gas released per unit surface area of PSA sheet
(in .mu.g/cm.sup.2). The same measurement method is employed in the
working examples described later.
[0131] The PSA sheet disclosed herein preferably has a 180.degree.
peel strength (adhesive strength) to stainless steel of about 3
N/20 mm or greater when determined based on JIS Z 0237:2009. The
PSA sheet having such adhesive strength can adhere well to an
adherend and provide good sealing. The adhesive strength is more
preferably 5 N/20 mm or greater, or yet more preferably about 7
N/20 mm or greater. The maximum adhesive strength is not
particularly limited. From the standpoint of preventing leftover
adhesive residue, it is suitably about 20 N/20 mm or less (e.g.
about 15 N/20 mm or less).
[0132] The adhesive strength of a PSA sheet is determined by the
following method: A PSA sheet subject to measurement is cut to a 20
mm wide, 100 mm long size to prepare a test piece. In an
environment, at 23.degree. C. and 50% RH, the adhesive face of the
test piece is press-bonded to a stainless steel plate (SUS304BA
plate) to obtain a measurement sample. The press-bonding is carried
out by rolling a 2 kg roller back and forth once. The measurement
sample is left standing in an environment at 23.degree. C. and 50%
RH for 30 minutes. Subsequently, using a tensile tester, based on
JIS Z 0237:2009, the peel strength (N/20 mm) is determined at a
tensile speed of 300 mm/min at a peel angle of 180.degree.. As the
tensile tester, Precision Universal Tensile Tester Autograph AG-IS
50N available from Shimadzu Corporation or a comparable product can
be used. The same measurement method is employed in the working
examples described later.
[0133] The PSA sheet disclosed herein preferably shows a
displacement less than about 5 mm in a shear holding power test
carried out with a 1 kg load at 60.degree. C. for one hour. The PSA
sheet satisfying this property shows good holding power even when
used at a relatively high temperature. The displacement in the
shear holding power test is more preferably less than about 3 mm,
or yet more preferably less than about 2 mm.
[0134] The shear holding power of a PSA sheet is determined by the
following method: In particular, the PSA sheet subject to
measurement is cut 10 mm wide, 20 mm long to prepare a test piece.
In an environment at 23.degree. C. and 50% RH, the adhesive face of
the test piece is press-bonded to a stainless steel plate to obtain
a measurement sample. The press-bonding is carried out by rolling a
2 kg roller back and forth once. The measurement sample is
vertically suspended and left in an environment at 60.degree. C.
and 50% RH for 30 minutes. Subsequently, a 1 kg weight is attached
to the free lower end of the test piece to start the test. The test
is carried out for one hour and the distance that the test piece
displaced (the displacement) is measured at one hour. The same
measurement method is employed in the working examples described
later.
[0135] The PSA sheet disclosed herein preferably has a tensile
modulus per unit width in a prescribed range. In particular, the
tensile modulus is preferably greater than 1000 N/cm, more
preferably greater than 1400 N/cm, yet more preferably greater than
1800 N/cm, or particularly preferably greater than 2200 N/cm. The
PSA sheet having such a tensile modulus has suitable rigidity and
is less susceptible to creasing. It tends to provide excellent
handling properties as well. The tensile modulus is preferably less
than 3500 N/cm, more preferably less than 3000 N/cm, or yet more
preferably less than 2800 N/cm (e.g. less than 2600 N/cm). The PSA
sheet having such a tensile modulus has good adherent
conformability and can well conform in a bent state to an area of
the adherend including a corner.
[0136] The tensile modulus per unit width of PSA sheet is
determined as follows: In particular, the PSA sheet is cut to a 10
mm wide, 50 mm long strip to prepare a test piece. The two ends of
the length of the test piece are clamped with chucks in a tensile
tester. In an atmosphere at 23.degree. C., at an inter-chuck
distance of 20 mm, at a speed of 50 mm/min, a tensile test is
conducted using the tensile tester to obtain a stress-strain curve.
Based on the initial slope of the resulting stress-strain curve,
the Young's modulus (N/mm.sup.2=MPa) is determined by linear
regression of the curve between two specified strain points
.epsilon.1 and .epsilon.2. From the product of the resulting value
and the thickness of the PSA sheet, the tensile modulus per unit
width (N/cm) can be determined. As the tensile tester, a commonly
known or conventionally used product can be used. For instance,
AUTOGRAPH AG-IS available from Shimadzu Corporation or a comparable
product can be used.
<Applications>
[0137] The PSA sheet disclosed herein has excellent moisture
resistance with reduced gas emission; and therefore, it is
preferably used in various applications where entry of moisture and
gas is desirably limited. For instance, the PSA sheet disclosed
herein is preferably used fir various electronic devices. More
specifically, it is preferably used as a sealing material (e.g. a
sealing material to seal its internal space) in the electronic
devices. In a preferable embodiment, for instance, the PSA sheet is
preferably used for sealing the internal space of a magnetic disc
device such as HDD. In this application, an included gas such as
siloxane gas may cause damage to the device; and therefore, it is
important to prevent such gas contamination. In a magnetic disc
device employing HAMR, it is important to prevent entrance of water
which badly affects the recording life. By using the PSA sheet
disclosed herein as a sealing material (or a cover seal) for a HAMR
magnetic disc device, a magnetic recording device having a higher
density can be obtained.
[0138] FIG. 4 shows an embodiment of the magnetic disc device as a
favorable example to which the art disclosed herein can be applied.
FIG. 4 shows a cross-sectional diagram schematically illustrating
the magnetic disc device according to an embodiment. A magnetic
disc device 100 comprises a data-recording magnetic disc 110, a
spindle motor 112 that rotates magnetic disc 110, a magnetic head
114 that reads and writes data on magnetic disc 110, and an
actuator 116 that supplies power to magnetic head 114. Actuator 116
has a built-in linear motor not shown in the drawing. In this
example of constitution, two magnetic discs 110 are included, but
it is not limited to this and three or more magnetic discs may be
included.
[0139] These components of magnetic disc device 100 are placed in a
housing 120 which serves as a casing for magnetic disc device 100.
In particular, the components of magnetic disc device 100 are
contained in a box-shaped housing base member (a support structure)
122 having a top opening and the top opening of housing base member
122 is covered with a rigid cover member 124. More specifically,
the top opening of housing base member 122 has a recessed portion
around the inner circumference and the outer rim of cover member
124 is placed on the bottom of recessed portion 126, with cover
member 124 covering the opening. A PSA sheet 101 is applied from
the top face of cover member 124 so as to entirely cover the cover
member 124 and the top face (outer circumference of the opening) of
housing 120, that is, the entire top face of housing 120,
altogether. This seals a space 140 present between housing base
member 122 and cover member 124 as well as other holes and void
spaces that communicate from the inside to the outside of magnetic
disc device 100, thereby keeping the inside of the device airtight.
Such a sealing structure using PSA sheet 101 as the sealing
material (cover seal) can be made thinner than a conventional
counterpart that uses a cover member and a gasket to obtain
air-tight properties. In addition, because it does not require the
use of a liquid gasket, outgassing from the gasket can be
eliminated as well. In this embodiment, the width of the top rim
(face of the frame) of housing base member 122 is about 0.1 mm to 5
mm (e.g. 3 mm or less, or even 2 mm or less) at its narrowest
portion, with the width being the distance between the outer
circumference and inner circumference of the top rim of housing
base member 122. When PSA sheet 101 is applied as a cover seal to
the top face of housing base member 122, the top rim of housing
base member 122 provides a bonding surface to PSA sheet 101,
forming a portion that isolates the internal space of magnetic disc
device 100 from the outside. According to the art disclosed herein,
even in an application where the width of bonding surface
(through-bonding-plane permeation distance) is limited, the
internal space can be maintained air tightly and dry
(moisture-resistant).
[0140] FIG. 5 shows another embodiment of the magnetic disc device
to which the art disclosed herein can be applied. A magnetic disc
device 200 has basically the same constitution as the embodiment
described above except for the way a PSA sheet 201 is applied.
Different features are described below. In magnetic disc device
200, PSA sheet 201 covers cover member 224 and the top face (outer
circumference of the opening) of housing base member 222
altogether, having a margin (or an extending portion) that further
extends to the side of housing 220. In particular, the extending
portion is bent from the top face over the corner of top rim to the
side of housing base member 222. The extending portion may be
provided entirely or partially at each side forming the outer
circumference of the top face of housing 220. In other words, in
magnetic disc device 200, PSA sheet 201 is applied, at least
partially covering the top and side faces of housing 220 in a U
shape. Similar to PSA sheet 101 according to the embodiment
described above, PSA sheet 201 seals a space 240 present between
housing base member 222 and cover member 224 as well as other holes
and void spaces that communicate from the inside to the outside of
magnetic disc device 200; and because it is applied with a margin
extending to the side of housing base member 222, the sealed state
is extended in the in-plane direction of bonding area. This results
in a larger distance (width) of the bonding area of PSA sheet 201
separating the outside and space 240, etc., and it inhibits
moisture permeation via the bonding area of PSA sheet 201, thereby
further enhancing the moisture resistance. In this embodiment, the
distance of PSA sheet 201 extending from the top rim (top edge of
the side) to the side of housing 220 (i.e. the length of PSA sheet
201 that covers the side (lateral surface)) is about 1 mm or
greater (e.g. 2 mm or greater, or even 3 mm or greater).
[0141] In these embodiments,cover members 124 and 224 cover
magnetic discs 110 and 210 as well as actuators 116 and 216
altogether, respectively, in one piece. However, they are not
limited to these. They may cover magnetic discs 110 and 210,
actuators 116 and 216, and other components, separately; or they
may not cover actuators 116 or 216 while covering magnetic discs
110 and 210. Even in these embodiments, by applying the PSA sheet
over the cover member, the inside of the device can be made
moisture-resistant and air-tight. In a magnetic disc device having
such an embodiment, the moisture resistance and air-tight
properties are obtained with the thin PSA sheet, thereby achieving
a thin sealing structure. This can increase the capacity for
housing magnetic discs, bringing about a magnetic disc device
having a higher density and a larger capacity.
[0142] Matters disclosed by this description include the following:
[0143] (1) A magnetic disc device comprising
[0144] at least one data-recording magnetic disc,
[0145] a motor that rotates the magnetic disc,
[0146] a magnetic head that at least either reads or writes data on
the magnetic disc,
[0147] an actuator that moves the magnetic head, and
[0148] a housing that houses the magnetic disc, the motor, the
magnetic head and the actuator; wherein
[0149] the housing is provided with a cover seal, the cover seal
being a PSA sheet comprising a moisture-impermeable layer and a PSA
layer provided to one face of the moisture-impermeable layer,
and
[0150] the PSA layer comprises a polymer A having a weight average
molecular weight of 5.times.10.sup.4 or higher as a base polymer
and a polymer B having a number average molecular weight of 1000 or
higher and a weight average molecular weight lower than
5.times.10.sup.4. [0151] (2) The magnetic disc device according to
(1) above, wherein the housing comprises a box-shaped housing base
member having a top opening and a cover member to cover the
opening. [0152] (3) The magnetic disc device according to (2)
above, wherein the housing base member has a recessed portion
around the inner circumference of the top opening and the outer rim
of the cover member is placed on the bottom of the recessed
portion. [0153] (4) The magnetic disc device according to any of
(1) to (3) above, wherein the cover member has a hole. [0154] (5)
The magnetic disc device according to any of (1) to (4) above,
wherein e PSA sheet seals the internal space of the magnetic disc
device. [0155] (6) The magnetic disc device according to any of (1)
to (5) above, wherein the PSA sheet covers and seals the top face
of the housing base member of the magnetic disc device. [0156] (7)
The magnetic disc device according to any of (1) to (6) above,
capable of heat-assisted magnetic recording. [0157] (8) A PSA sheet
comprising a moisture-impermeable layer and a PSA layer provided to
one face of the moisture-impermeable layer, wherein
[0158] the PSA layer comprises a polymer A having a weight average
molecular weight of 5.times.10.sup.4 or higher as a base polymer
and a polymer B having a number average molecular weight of 1000 or
higher and a weight average molecular weight lower than
5.times.10.sup.4. [0159] (9) The PSA sheet according to (8) above,
wherein the number average molecular weight of the polymer B is
2000 or higher. [0160] (10) The PSA sheet according to (8) or (9)
above, wherein the polymer B is at least one species selected among
olefinic polymers and diene-based polymers. [0161] (11) The PSA
sheet according to any of (8) to (10) above, wherein the polymer B
is a polybutene. [0162] (12) The PSA sheet according to any of (8)
to (11) above, wherein the polymer A is at least one species
selected among rubber-based polymers and acrylic polymers. [0163]
(13) The PSA sheet according to any of (8) to (12) above, having a
moisture permeability lower than 30 .mu.g/cm.sup.224 h in in-plane
directions of bonding area of PSA sheet, determined at a permeation
distance of 2.5 mm based on the MOCON method. [0164] (14) The PSA
sheet according to any of (8) to (13) above, having an amount of
thermally released gas of 10 .mu.g/cm.sup.2 or less, determined at
130.degree. C. for 30 minutes by gas chromatography/mass
spectrometry. [0165] (15) The PSA sheet according to any of (8) to
(14) above, used for sealing the internal space of a magnetic disc
device. [0166] (16) A release liner-supported PSA sheet comprising
the PSA sheet according to any of (8) to (15) above and a release
liner protecting the adhesive face of the PSA sheet, wherein the
release liner is a non-silicone-based release liner free of a
silicone-based release agent. [0167] (17) A magnetic disc device
comprising the PSA sheet according to any of (8) to (14) above.
[0168] (18) The magnetic disc device according to (17) above,
wherein the PSA sheet seals the internal space of the magnetic disc
device. [0169] (19) The magnetic disc device according to (17) or
(18) above, wherein the magnetic disc device has a housing base
member and the PSA sheet is a cover seal that covers and seals the
top face of the housing base member. [0170] (20) The magnetic disc
device according to any of (17) to (19) above, capable of
heat-assisted magnetic recording.
EXAMPLES
[0171] Several working examples related to the present, invention
are described below, but the present invention is not intended to
be limited to these examples. In the description below, "parts" and
"%" are by weight unless otherwise specified.
Example 1
(Preparation of Moisture-Impermeable Layer)
[0172] By dry bonding lamination, were laminated 25 .mu.m thick PET
film (PET layer) as the first resin layer, 7 .mu.m thick aluminum
foil (Al layer) as the inorganic layer and 9 .mu.m thick PET film
(PET layer) as the second resin layer in this order from the front
(outer surface side) to the backside (PSA layer side). Between each
resin layer and the inorganic layer, was laminated a 3 .mu.m thick
adhesive layer. A 47 .mu.m thick moisture-impermeable layer was
thus prepared.
(Preparation of PSA Composition)
[0173] In toluene, were dissolved 50 parts of butyl rubber (IIR:
butyl rubber available from JSR, product name JSR BUTYL 268,
Mw.about.54.times.10.sup.4, Mw/Mn.about.4.5) as the base polymer
and polybutene (NISSEKI POLYBUTENE HV-1900 available from JXTG
Nippon Oil & Energy Corporation, Mn 2900) to prepare a PSA
composition with 25% NV.
(Fabrication of PSA Sheet)
[0174] The PSA composition obtained above was applied to one face
(the second resin layer-side surface) of the moisture-impermeable
layer to have a thickness of 30 .mu.m after dried, and allowed to
dry at 120.degree. C. for 3 minutes to form a PSA layer. A PSA
sheet was thus obtained according to this Example. For protection
of the surface (adhesive face) of the PSA layer, was used a release
liner formed of thermoplastic film treated with release agent
(product name HP-S0 available from Fujico Co. Ltd.; 50 .mu.m
thick).
Examples 2, 3 and Reference Example
[0175] In place of the polybutene in Example 1, were used NISSEKI
POLYBUTENE HV-300 (Mn 1400) in Example 2 and NISSEKI POLYBUTENE
HV-15 (Mn 630) in Example 3, both available from JXTG Nippon Oil
& Energy Corporation. Otherwise in the same manner as Example
1, were obtained PSA sheets according to Examples 2 and 3. Without
using the polybutene, but otherwise basically in the same manner as
Example 1, was obtained a PSA sheet according to Reference
Example.
[Moisture Permeability (Cup Method) of PSA Layer]
[0176] The moisture permeability in the thickness direction of each
PSA layer was determined based on the water vapor permeability test
(cup method) in JIS Z 0208. In particular the PSA composition was
applied to a releasable surface and allowed to dry to form a 50
.mu.m thick PSA layer. The PSA layer was adhered to 2 .mu.m thick
PET film (DIAFOIL available from Mitsubishi Plastics, Inc.) with
rubber roller. The PET layer-supported PSA layer was cut to a
circle of 30 mm diameter to fit the diameter of the test cup (an
aluminum cup of 30 mm diameter used in the cup method of JIS Z
0208). This was used as a test sample. A prescribed amount of
calcium chloride was placed in the cup and the opening of the cup
was sealed with the test sample prepared above. The cup covered
with the test sample was placed in a thermostat wet chamber at
60.degree. C. and 90% RH and left standing for 24 hours. The change
in weight of calcium chloride before and after this step was
determined to obtain the moisture permeability (g/cm.sup.224
h).
[0177] For each Example, Table 1 shows the species of PSA as well
as the test results of moisture permeability (cup method)
(g/cm.sup.224 h), storage moduli G'(25.degree. C.) (MPa),
through-bonding-plane moisture permeability of PSA sheet
(.mu.g/cm.sup.224 h), adhesive strength (N/20 mm), shear holding
power (mm) and amount of thermally released gas
(.mu.g/cm.sup.2).
[Table 1]
TABLE-US-00001 [0178] TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ref. Ex. PSA
Polymer A IIR IIR IIR IIR Composition Polymer B HV-1900 HV-300
HV-15 -- Mn 2900 1400 630 -- Blend ratio(A/B) 50/50 50/50 50/50 100
G'(25.degree. C.) of PSA layer (Mpa) 0.2 0.15 0.1 0.3 Moisture
permeability 1.3 1.3 1.3 1.3 (cup method) (g/cm.sup.2 24 hr)
Moisture permeability in in- 17.4 24.3 87.0 30 plane directions of
bonding area (.mu.g/cm.sup.2 24 h) Adhesive strength 7.8 7.2 4.9
2.6 (N/20 mm) Shear holding power (mm) 1.6 3.2 falling 0.2 Amount
of thermally 0.9 2.2 10.5 0.8 released gas (.mu.g/cm.sup.2)
[0179] As shown in Table 1, as compared to Reference Example not
using a polymer B, Examples 1 and 2 showed lower moisture
permeation in in-plane directions of bonding area, with both
Examples using a PSA layer comprising a high-molecular-weight
polymer A as a base polymer and a low-molecular-weight polymer B
with Mn.gtoreq.1000. On the other hand, in Example 3 using a
polymer B with Mn<1000, the moisture resistance did not improve.
The PSA sheet of Example 3 failed the shear holding power test and
fell off during the test. On the other hand, in Examples 1 and 2,
the displacement in the shear holding power test was at or below
the prescribed value, showing good holding power. The PSA sheets
according to Examples 1 and 2 tended to thermally release less gas
than the PSA sheet of Example 3. Especially, Example 1 using a
polymer B with Mn.gtoreq.2000 performed best in all tests
(through-bonding-plane moisture permeability adhesive strength,
shear holding power, and amount of thermally released gas) among
the examples tested (Examples 1 to 3). It is noted that in the
evaluation of adhesive strength, Example 3 showed leftover adhesive
residue after peeled off.
[0180] Although specific embodiments of the present invention have
been described in detail above, these are merely for illustrations
and do not limit the scope of claims. The art according to the
claims includes various modifications and changes made to the
specific embodiments illustrated above.
REFERENCE SIGNS LIST
[0181] 1, 101, 201 PSA sheets [0182] 10 moisture-impermeable layer
[0183] 12 resin layer [0184] 14 inorganic layer [0185] 16 second
resin layer [0186] 20 PSA layer [0187] 50 moisture permeability
tester [0188] 52 wet chamber [0189] 54 dry chamber [0190] 56 metal
plate [0191] 58 opening (in metal plate) [0192] 60 measurement
sample [0193] 100, 200 magnetic disc devices [0194] 110, 210
magnetic discs [0195] 112, 212 spindle motors [0196] 114, 214
magnetic heads [0197] 116, 216 actuator [0198] 120, 220 housing
[0199] 122, 222 housing base member [0200] 124, 224 cover member
[0201] 120, 220 recessed portions [0202] 140, 240 spaces
* * * * *